An excellent post was made in the old RBY tiers thread about which Pokemon should be Top Tier:
http://www.smogon.com/forums/showpost.php?p=4262078&postcount=29
This guy has it spot on. Squirtle, Charmander, Nidos, and Sandshrew can all solo the game. Granted, they all require X Accuracy (and later X Speed) to OHKO their opponents, but they're all bulky enough to set up on opponents - so it's not like you have to get lucky to do this. Seel (specifically, the traded one) gets Top Tier as well despite being a late add - because it levels up so quickly, and is also bulky enough to set up and Horn Drill the E4. Finally, Zapdos gets top tier because it smashes the E4 with Thunderbolt.
Compare that to our top tier here:
Abra - Only belongs in the top tier if you can trade it to become Alakazam (many of us can't or don't bother), and even then it should be more specifically the Abra in Goldenrod (EDIT: Celadon), since it only needs to gain one level. Getting an Alakazam in Goldenrod (EDIT: Celadon) makes the rest of the game extremely easy, but it's not like it's any "better" at soloing the E4 than the guys above. I'd go with Abra (with trade) in Top Tier, Abra (no trade) at the top of the High Tier.
Articuno - Significantly more annoying to catch than Zapdos, and it doesn't solo the E4 (Lorelei and Gary, esp in Y). Yeah, it's good against the gyms, but that seems like a poor reason to put it in the Top Tier. This is another Pokemon that belongs near the top of the High Tier.
Bulbasaur - This is a travesty. Every game has one starter that is weaker than the others, and frankly it's more fun to start with that one since it provides more of a "challenge." (not really in RB) Bulbasaur is the obvious choice - he's great in the beginning, but then he can't handle Sabrina and Blaine without help; at the end, even an overleveled Venusaur struggles against Lorelei, Agatha, and Gary. At best, he belongs in the middle of the High Tier.
Diglett - I'll make this short: Ground STAB is a very nice thing to have in this game, but at the end of the game he's definitely outclassed by Sandslash. The big difference is that his poor defense makes it impossible for him to set up for OHKOs. This means he struggles against most of the E4 and Gary, something that is decidedly untrue for Sandlash. This guy belongs near the top of the High Tier.
Only Sandshrew is missing from the Top Tier, and he belongs there for all the reasons that Dugtrio does except that he's much better at the end of the game.
Summary:
1) Move Sandshrew to the Top Tier
2) Move Abra (no trade), Dugtrio, and Articuno to the top of the High Tier
3) Move Bulbasaur to the middle of the High Tier
4) If you're willing to create a separate entry for Abra (trade), then it justifiably belongs in the Top Tier.
Aside: Needless to say, only RB Squirtle and Charmander belong in the Top Tier.
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EDIT
It's come to my attention that X-items are not allowed for consideration of tiering. I don't agree with this: OHKO Pokemon that are bulky enough to set up with X Speed and X Accuracy deserve to be in the highest tier because they win the game as efficiently as possible. It's not like you need to spend extra time (i.e. overleveling Pokemon) and you obviously have plenty of money to buy the items by the time you actually need them (i.e. Elite Four and Gary). Stupid arguments like "X Items can make any Pokemon good enough" are garbage because most Pokemon aren't bulky enough to set up before they get KOed. The idea is to take luck out of the picture as much as possible - and Top Tier Pokemon consistently set up with X Speed and X Accuracy before OHKOing the entire opposing team. Besides, they're good enough to STAB everything before the E4 and Gary - so they're High Tier even before considering OHKOs.
This should be the definition of Top Tier: Pokemon that can consistently win the game without support from any other Pokemon (and without going out of your way to level up). This should be the definition of High Tier: Pokemon that can consistently defeat the vast majority of opponents, but require help from other Pokemon for specific matchups. You need a team of 2-4 High Tier Pokemon to beat the game, or you can just use a Top Tier Pokemon and five Magikarps.
Nevertheless, if those are the thread rules, then Sandshrew is probably out of the Top Tier and into the top of the High Tier. He still belongs above Dugtrio, because of his superior Attack and Defense. Seel stays in the Top Tier because it crushes the last 25% of the game with STAB Surf and Blizzard. Abra (no trade) is probably a Top Tier under these conditions because Kadabra is excellent against everything beside Lance. Articuno still belongs in the High Tier because it's strictly inferior to Zapdos. Bulbasaur also belongs in the High Tier because it's strictly inferior to the other two starters.
http://www.smogon.com/forums/showpost.php?p=4262078&postcount=29
This guy has it spot on. Squirtle, Charmander, Nidos, and Sandshrew can all solo the game. Granted, they all require X Accuracy (and later X Speed) to OHKO their opponents, but they're all bulky enough to set up on opponents - so it's not like you have to get lucky to do this. Seel (specifically, the traded one) gets Top Tier as well despite being a late add - because it levels up so quickly, and is also bulky enough to set up and Horn Drill the E4. Finally, Zapdos gets top tier because it smashes the E4 with Thunderbolt.
Compare that to our top tier here:
Abra - Only belongs in the top tier if you can trade it to become Alakazam (many of us can't or don't bother), and even then it should be more specifically the Abra in Goldenrod (EDIT: Celadon), since it only needs to gain one level. Getting an Alakazam in Goldenrod (EDIT: Celadon) makes the rest of the game extremely easy, but it's not like it's any "better" at soloing the E4 than the guys above. I'd go with Abra (with trade) in Top Tier, Abra (no trade) at the top of the High Tier.
Articuno - Significantly more annoying to catch than Zapdos, and it doesn't solo the E4 (Lorelei and Gary, esp in Y). Yeah, it's good against the gyms, but that seems like a poor reason to put it in the Top Tier. This is another Pokemon that belongs near the top of the High Tier.
Bulbasaur - This is a travesty. Every game has one starter that is weaker than the others, and frankly it's more fun to start with that one since it provides more of a "challenge." (not really in RB) Bulbasaur is the obvious choice - he's great in the beginning, but then he can't handle Sabrina and Blaine without help; at the end, even an overleveled Venusaur struggles against Lorelei, Agatha, and Gary. At best, he belongs in the middle of the High Tier.
Diglett - I'll make this short: Ground STAB is a very nice thing to have in this game, but at the end of the game he's definitely outclassed by Sandslash. The big difference is that his poor defense makes it impossible for him to set up for OHKOs. This means he struggles against most of the E4 and Gary, something that is decidedly untrue for Sandlash. This guy belongs near the top of the High Tier.
Only Sandshrew is missing from the Top Tier, and he belongs there for all the reasons that Dugtrio does except that he's much better at the end of the game.
Summary:
1) Move Sandshrew to the Top Tier
2) Move Abra (no trade), Dugtrio, and Articuno to the top of the High Tier
3) Move Bulbasaur to the middle of the High Tier
4) If you're willing to create a separate entry for Abra (trade), then it justifiably belongs in the Top Tier.
Aside: Needless to say, only RB Squirtle and Charmander belong in the Top Tier.
--------------------------------------------------------------------------------------------
EDIT
It's come to my attention that X-items are not allowed for consideration of tiering. I don't agree with this: OHKO Pokemon that are bulky enough to set up with X Speed and X Accuracy deserve to be in the highest tier because they win the game as efficiently as possible. It's not like you need to spend extra time (i.e. overleveling Pokemon) and you obviously have plenty of money to buy the items by the time you actually need them (i.e. Elite Four and Gary). Stupid arguments like "X Items can make any Pokemon good enough" are garbage because most Pokemon aren't bulky enough to set up before they get KOed. The idea is to take luck out of the picture as much as possible - and Top Tier Pokemon consistently set up with X Speed and X Accuracy before OHKOing the entire opposing team. Besides, they're good enough to STAB everything before the E4 and Gary - so they're High Tier even before considering OHKOs.
This should be the definition of Top Tier: Pokemon that can consistently win the game without support from any other Pokemon (and without going out of your way to level up). This should be the definition of High Tier: Pokemon that can consistently defeat the vast majority of opponents, but require help from other Pokemon for specific matchups. You need a team of 2-4 High Tier Pokemon to beat the game, or you can just use a Top Tier Pokemon and five Magikarps.
Nevertheless, if those are the thread rules, then Sandshrew is probably out of the Top Tier and into the top of the High Tier. He still belongs above Dugtrio, because of his superior Attack and Defense. Seel stays in the Top Tier because it crushes the last 25% of the game with STAB Surf and Blizzard. Abra (no trade) is probably a Top Tier under these conditions because Kadabra is excellent against everything beside Lance. Articuno still belongs in the High Tier because it's strictly inferior to Zapdos. Bulbasaur also belongs in the High Tier because it's strictly inferior to the other two starters.