Why is this thread so empty? Lets contribute a little:
RBY
Probably the Mon I use the mosy in RBY after Big 4 and maybe Jynx (best lead). Not that I don,t like Starmie or Alakazam... well, in the case of Starmie I kinda truly don,t like it, always useless in my hands and broken against me.
Back to Rhydon, it has huge power, flawless coverage just between STABs, 101 Subs, explosion absorbtion and it makes Zapdos totally useless. Its just so good, especially with Para support. Right now, I probably have more (good) RBY teams with Rhydon than without it.
GSC
A flexible Mon that has almost no reliable counters. It can do everything for a team, statusing, stealing an item, exploding, coming carefully on a Electric/Snorlax/Machamp/Ground/Vaporeon, etc. . Like the Big 4 of this Gen, its a Mon that is never useless.
ADV
A Mon with potential to clean entire teams and save otherwise lost battles with some luck. I like Mons like this one. It outspeeds everything, its powerful, it has perfect coverage and even the sturdiest phisical walls can end up losing to the 30% flinch chance. It has just one set (well, kinda. Will definetely have just one if DrumPass gets banned. Sub-Pressure sets don,t count) but that one is all Aero needs.
DPP
Best Scarfer of the game. Thats already enough for me, as I tend to struggle choosing the scarfers. It has U-Turn (the one move that separates good Scarfer from bad ones), it resists hazards, it hits hard and its bulk is not that bad for an offensive Mon. Also, there are plenty of other sets to choose from if for some reason I decide to use another Scarfer.
BW
I don,t like defensive Drill at all as a Spinner in BW. Same for defensive Starmie, which even loses to the one good spinblocker. Offensive versions of both Mons are better but not as reliable. Forretress is terrible, and while Donphan is decent, its hard to fit with so many good Ground types (see: Chomp, Lando, Hippo, Gliscor, etc.) in this Meta. So, what is left? Tenta! And he is amazing and reliable at this role. People tend to discard Tenta as a Rain only Mon, but he is equally good on Weatherless, Sand, Hail and even Sun. In every situation it helps a lot against opposing Rain, which is really common. And if the opponent doesn,t bring Rain... well, Tenta will still have some turns to Spin, put Tspikes or status something, you just have to play offensively with the rest of the team. Every single (Bad) tiering decision has been making Tentacruel better, as now he doesn,t have to worry about being trapped, being set-up fodder for Venu in Sun or for BP teams. In a gen in which Spikes are broken, having a great easy to use spinner like this one is always useful.
ORAS
The balanced core I tend to use to cover half of the Meta in 2 slots. This is the one (Old) Gen where defensive Drill is good, as there is almost no Rain and Fairies now exist. With Clefable being S Rank and a common rocker even after Sableye got banned, Drill shines in this Meta. It will always find a chance to either set Rocks or remove them and Gyarados abuses those Rocks with Roar. Speaking of Gyarados, his defensive typing, like Drill one, is really underrated. He checks Heatran, both Zards, Keldeo, Lopunny, Volcarona, Swampert and Talonflame among others. If its Mega defensive, it loses the ability to check some of the above, but gains it against Bisharp, Weavile, Crawdaunt and will start to take Electric Moves. It absorbs Status and it will take hits from dangerous set-up Mons like Zard-X, Manaphy, Serperior, Suicune, Tbolt Less Mega Latias and Clefable, easily phazing them away.
Drill and Gyara both have amazing defensive sinergy, but can function individually. And of course, both have some other Sets, with Gyarados having a good Mega DDance set and Drill being a Sand Sweeper, where it still can use Leftovers to last longer.