One of the more interesting things about Blue is that it has two Unaware walls that are actually fairly complimentary and, with the appropriate stats, quite competent -Quagsire and Swoobat. It has a number of solid leaders for bolstering their bulk, too. Probably the overall strongest at assisting them at being Unaware walls per se is Suicune, but it's notable to me that Garchomp is actually a direct boost to every bulk-related stat on both 'mons! It's not even that far behind Suicune. As such, Garchomp can potentially lead a Blue-heavy team and actually provide very solid walls at the same time as giving you solid attackers -it's also basically a direct upgrade to regular Gyarados' statline (Somewhat less Special Defense, but the HP advantage blunts it), basically a direct upgrade to Feraligatr's statline (Slightly lower Defense, but the HP advantage overcomes it), a direct upgrade to Heracross' statline, basically a direct upgrade to even Mega Altaria's statline (worse non-HP defenses, but the HP advantage makes it essentially equal in bulk. The loss in Special Attack is more meaningful, but Mega Altaria usually runs primarily Physical anyway), a direct upgrade to the Shell Smashing Huntail, a direct upgrade to Lucario (Except on Special Attack)...
Garchomp is a fantastic donor all around. Some other donors might provide better bulk, and if you want Special Attack there's definitely better (Thundurus-Therian gifting its 145 Special Attack to Swellow for Boomburst shenanigans sounds pretty cool -it even has Scrappy to break Ghosts! Focus Energy is its only boosting, unfortunately), but Garchomp all by itself can bring strong, flexible Blue teams to the table, and it's not a problem to bring in something like Gengar to cover your Special attacking needs -or run something like Simple Swoobat, relying on Simple Stored Power Calm Mind backed by having raised your Special Attack 3 points and gained a bunch of bulk. (Lost some Speed, admittedly)
Garchomp can basically lead an entire team all by itself to good effect, is what I'm saying.
Red, unfortunately, doesn't have an equivalently impressive dynamic. Latias is probably it's overall best leader, but Red contains a number of powerful Physical attackers that resent having their Attack slashed to 80. Scizor and Darmanitan are your main options for supporting the like of Crawdaunt -both of them are basically 100% improvements over Crawdaunt (Scizor has worse Special Attack, but who cares? Darmanitan has worse Defense, but higher HP to partially offset the loss. Scizor is probably the overall better choice for supporting Crawdaunt) including boosting its already impressive Attack. Talonflame with 140 Attack is another pretty terrifying idea, and it doesn't mind dropping to Darmanitan's Speed tier most of the time. Darmanitan's HP advantage even helps it shrug off recoil damage from Brave Bird.
Red struggles to get real walls, though. Latias and, weirdly enough, Entei are some of your best donors for overall bulk. Throh offers more, but Latias and Entei are probably going to weigh you down less, and their advantages in Special Attack and Speed make them more likely to be able to support a Red-heavy team in proper diversity. Porygon-Z is your overall most powerful Special attacker donor in Red, but the movepools and Abilities of Red Pokemon mostly slant toward supporting Physical. It's pretty much a direct improvement to the Rotom family, anyway, so if you like running Porygon-Z alongside a Rotom in OU, that's cool.
Weirdly enough, in Green your best overall wall-stats donor is probably Tyranitar. 100/110/100 defensive stats is solid, and there's not a lot of competition. Florges is solid if you want Special bulk on your team that isn't as painfully vulnerable to Physical as a Blissey team, and Zygarde has even more Physical bulk than Tyranitar and more or less the same Special bulk (Less Special Defense, more HP), but Tyranitar's low Speed is generally a non-issue for walls and the potential for pressuring enemies with its high Attack and/or passable Special Attack is pretty good.
That Tyranitar is an excellent Pokemon by itself and a good wall donor is convenient, because Green has a lot of Pokemon that make a stab at being walls but normally lack the stats to really support it, like Cradily, Flygon (It's a Defogger that laughs at all hazards), Poison Heal versions of Breloom (Or, really, any form of Breloom appreciates inheriting from Tyranitar -you lose 5 Speed to get better in every other area!), and others beside. It also allows for some interesting options -Tornadus-Therian with Tyranitar's statline replaces a fast U-Turning Regenerator of okay bulk with a slow-turner that is much more brutal on the Physical end and has noticeably more bulk, which has all kinds of utility itself. Whimsicott is happy to inherit from Tyranitar, for instance, as the only thing it loses is Speed and usually it's relying so heavily on Prankster this isn't much of a loss.
Yellow is a color of extremes. It has one of the hardest-hitting Physical attackers in OU (Haxorus), one of the most overall bulky Pokemon to ever grace OU (Cresselia), it has the fastest OU-legal Pokemon ever (Ninjask), the Pokemon with the highest non-HP defensive stats (Shuckle), and Keldeo is legitimately one of the harder-hitting Pokemon the Special side. Jolteon is also insanely fast and offers a Special-oriented option for just ludicrous Speed, and it's less fragile too. This means Yellow can tailor itself to a fairly impressive degree, though Jirachi is the closest thing it has to an "extremely general choice" for donor.
One of the more infuriating things it can do is Cresselia gracing Simple Numel with its 120/120/130 defensive stats for utterly maddening bulk.
+1 252 Atk Mold Breaker Haxorus Earthquake vs. +2 252 HP / 252+ Def Numel (Cresselia's stats): 114-136 (25.6 - 30.6%) -- 2% chance to 4HKO after Leftovers recovery
Yeah, Numel can literally set up in Haxorus' face. And then Rest, not even needing Sleep Talk.
Typhlosion borrowing Keldeo's stats is a pretty straightforward upgrade to Eruption sets, or you could run something like Raikou to edge out more Speed tiers. Beautifly and Mothim might also appreciate such, as they're Quiver Dancers that might be relevant with those kinds of stats. Maybe. Okay, probably not, but I can dream. Persian with Haxorus' stats actually sounds kind of decent.
252 Atk Technician Haxorus (Normal type) Fake Out vs. 0 HP / 0 Def Mew: 108-127 (31.6 - 37.2%) -- 83.8% chance to 3HKO
That's a pretty obnoxious amount of damage Persian gets in for free. You've also got Feint to then follow-up with if you desperately need to be able to revenge priority abusers, or you can simply U-Turn out to hold momentum. The damage output is pretty solid at pressuring even walls, even when you account for the fact that a Physically Defensive Mew statline actually takes no more than about 25%. That's still nearly 50% from just Fake Out+U-Turn. You've also got utility moves like Knock Off to clear out items and, of course, bring the pain to Ghosts foolish enough to think that being a Normal type means they can switch safely into you.
Yellow has a lot of interesting things it can do, all-around. A color of extremes.
Garchomp is a fantastic donor all around. Some other donors might provide better bulk, and if you want Special Attack there's definitely better (Thundurus-Therian gifting its 145 Special Attack to Swellow for Boomburst shenanigans sounds pretty cool -it even has Scrappy to break Ghosts! Focus Energy is its only boosting, unfortunately), but Garchomp all by itself can bring strong, flexible Blue teams to the table, and it's not a problem to bring in something like Gengar to cover your Special attacking needs -or run something like Simple Swoobat, relying on Simple Stored Power Calm Mind backed by having raised your Special Attack 3 points and gained a bunch of bulk. (Lost some Speed, admittedly)
Garchomp can basically lead an entire team all by itself to good effect, is what I'm saying.
Red, unfortunately, doesn't have an equivalently impressive dynamic. Latias is probably it's overall best leader, but Red contains a number of powerful Physical attackers that resent having their Attack slashed to 80. Scizor and Darmanitan are your main options for supporting the like of Crawdaunt -both of them are basically 100% improvements over Crawdaunt (Scizor has worse Special Attack, but who cares? Darmanitan has worse Defense, but higher HP to partially offset the loss. Scizor is probably the overall better choice for supporting Crawdaunt) including boosting its already impressive Attack. Talonflame with 140 Attack is another pretty terrifying idea, and it doesn't mind dropping to Darmanitan's Speed tier most of the time. Darmanitan's HP advantage even helps it shrug off recoil damage from Brave Bird.
Red struggles to get real walls, though. Latias and, weirdly enough, Entei are some of your best donors for overall bulk. Throh offers more, but Latias and Entei are probably going to weigh you down less, and their advantages in Special Attack and Speed make them more likely to be able to support a Red-heavy team in proper diversity. Porygon-Z is your overall most powerful Special attacker donor in Red, but the movepools and Abilities of Red Pokemon mostly slant toward supporting Physical. It's pretty much a direct improvement to the Rotom family, anyway, so if you like running Porygon-Z alongside a Rotom in OU, that's cool.
Weirdly enough, in Green your best overall wall-stats donor is probably Tyranitar. 100/110/100 defensive stats is solid, and there's not a lot of competition. Florges is solid if you want Special bulk on your team that isn't as painfully vulnerable to Physical as a Blissey team, and Zygarde has even more Physical bulk than Tyranitar and more or less the same Special bulk (Less Special Defense, more HP), but Tyranitar's low Speed is generally a non-issue for walls and the potential for pressuring enemies with its high Attack and/or passable Special Attack is pretty good.
That Tyranitar is an excellent Pokemon by itself and a good wall donor is convenient, because Green has a lot of Pokemon that make a stab at being walls but normally lack the stats to really support it, like Cradily, Flygon (It's a Defogger that laughs at all hazards), Poison Heal versions of Breloom (Or, really, any form of Breloom appreciates inheriting from Tyranitar -you lose 5 Speed to get better in every other area!), and others beside. It also allows for some interesting options -Tornadus-Therian with Tyranitar's statline replaces a fast U-Turning Regenerator of okay bulk with a slow-turner that is much more brutal on the Physical end and has noticeably more bulk, which has all kinds of utility itself. Whimsicott is happy to inherit from Tyranitar, for instance, as the only thing it loses is Speed and usually it's relying so heavily on Prankster this isn't much of a loss.
Yellow is a color of extremes. It has one of the hardest-hitting Physical attackers in OU (Haxorus), one of the most overall bulky Pokemon to ever grace OU (Cresselia), it has the fastest OU-legal Pokemon ever (Ninjask), the Pokemon with the highest non-HP defensive stats (Shuckle), and Keldeo is legitimately one of the harder-hitting Pokemon the Special side. Jolteon is also insanely fast and offers a Special-oriented option for just ludicrous Speed, and it's less fragile too. This means Yellow can tailor itself to a fairly impressive degree, though Jirachi is the closest thing it has to an "extremely general choice" for donor.
One of the more infuriating things it can do is Cresselia gracing Simple Numel with its 120/120/130 defensive stats for utterly maddening bulk.
+1 252 Atk Mold Breaker Haxorus Earthquake vs. +2 252 HP / 252+ Def Numel (Cresselia's stats): 114-136 (25.6 - 30.6%) -- 2% chance to 4HKO after Leftovers recovery
Yeah, Numel can literally set up in Haxorus' face. And then Rest, not even needing Sleep Talk.
Typhlosion borrowing Keldeo's stats is a pretty straightforward upgrade to Eruption sets, or you could run something like Raikou to edge out more Speed tiers. Beautifly and Mothim might also appreciate such, as they're Quiver Dancers that might be relevant with those kinds of stats. Maybe. Okay, probably not, but I can dream. Persian with Haxorus' stats actually sounds kind of decent.
252 Atk Technician Haxorus (Normal type) Fake Out vs. 0 HP / 0 Def Mew: 108-127 (31.6 - 37.2%) -- 83.8% chance to 3HKO
That's a pretty obnoxious amount of damage Persian gets in for free. You've also got Feint to then follow-up with if you desperately need to be able to revenge priority abusers, or you can simply U-Turn out to hold momentum. The damage output is pretty solid at pressuring even walls, even when you account for the fact that a Physically Defensive Mew statline actually takes no more than about 25%. That's still nearly 50% from just Fake Out+U-Turn. You've also got utility moves like Knock Off to clear out items and, of course, bring the pain to Ghosts foolish enough to think that being a Normal type means they can switch safely into you.
Yellow has a lot of interesting things it can do, all-around. A color of extremes.