Nominations of what I think should go
Drizzle-No need to explain, there are already way to many pages for and against to need to go further indepth but it breaks a ridiculous number of pokemon, boosts their stab into a double stab and cause things like Ludicolo/Kabutops/Kingdra to seriously be considered broken which they aren't. It makes Manaphy a god and even after banning the "trio" it will still have Belly Drum Swift Swim Poliwrath, Shell Breakers (slightly more difficult to use but isn't impossible, especially with Screen support) in Gorebyss/Huntail/Omastar (Gorebyss/Huntail can also pass), Qwilfish, Floatzel, etc etc etc.
Manaphy-This+Rain=Broken. This in Non-rain=still almost broken just not as much because it can't abuse Hydration-Rest. But with support options like Screens, Acid Armor, etc, this thing can still be incredibly difficult to kill and has +3 Tail Glows as well. It can set up it's own Rain and team and 100 stats across the board is still phenomenal. And if Drizzle is staying (heaven forbid), this can not.
Landlos-High Speed, Unpredictablitly, Insane Power. It is able to be used in any weather and Sand gives it a fairly strong boost. It can Rock Polish/Swords Dance/Bulk Up/Calm Mind and can attack on both sides VERY hard. In Dream World it has Sheer Force/Encourage which allows it to abuse Calm Mind Earth Powers and Focus Blasts with no Life Orb recoil. Rock Polish allows it to outspeed nearly everything (except Floatzel in Rain or Shell Smash Swift Swimmer) and finish off weakened things and with the Sand Power boost easily finish off weakened things. Swords Dance is ridiculous power. It can ohko physical defensive Skarmoy with Stone Edge in Sand and does massive damage to everything. Bulk Up gives surviveability and makes it harder to physically kill and it isn't weak to any common priority attacks (except Ice Shard which rarely ohko anyways except from Mamoswine and after Bulk Up do even worse and Landlos could actually survive Choice Band Mamoswine with some Defense after Bulk Up).
Terakion-Similar to Landlos with High speed and power and semi-unpredictability since it too can abuse Swords Dance/Rock Polish. Can't specially attack but it does heavy damage and Rock Polish allows it to kill the fast things while Swords Dance can allow it to kill the walls. In Sandstorm it gets a special defensive boost which can help allow it to survive some weaker special attacks and is again, really quite fast and strong.
Drought-Might as well since it'll probably become even stronger if Drizzle's gone and has some scary monsters of its own like Ulgamoth, Heatran, Charizard, Venasaur, Victreebel, Shiftry, Mebjuka, etc. The only reason it isn't used as much because it got the short end of the stick in weather inducer.
Latios-Insane speed and special attack and it's Draco Meteors hit like a truck and is still annoying to switch in on since little can survive double Meteors besides a steel or something incredibly special bulky and I mean incredible (who rarely have things like Slack off).
Honorable Mentions-Mew, Doryuzu, Deoxys-D, Ulgamoth (eh why not?)
Drizzle-No need to explain, there are already way to many pages for and against to need to go further indepth but it breaks a ridiculous number of pokemon, boosts their stab into a double stab and cause things like Ludicolo/Kabutops/Kingdra to seriously be considered broken which they aren't. It makes Manaphy a god and even after banning the "trio" it will still have Belly Drum Swift Swim Poliwrath, Shell Breakers (slightly more difficult to use but isn't impossible, especially with Screen support) in Gorebyss/Huntail/Omastar (Gorebyss/Huntail can also pass), Qwilfish, Floatzel, etc etc etc.
Manaphy-This+Rain=Broken. This in Non-rain=still almost broken just not as much because it can't abuse Hydration-Rest. But with support options like Screens, Acid Armor, etc, this thing can still be incredibly difficult to kill and has +3 Tail Glows as well. It can set up it's own Rain and team and 100 stats across the board is still phenomenal. And if Drizzle is staying (heaven forbid), this can not.
Landlos-High Speed, Unpredictablitly, Insane Power. It is able to be used in any weather and Sand gives it a fairly strong boost. It can Rock Polish/Swords Dance/Bulk Up/Calm Mind and can attack on both sides VERY hard. In Dream World it has Sheer Force/Encourage which allows it to abuse Calm Mind Earth Powers and Focus Blasts with no Life Orb recoil. Rock Polish allows it to outspeed nearly everything (except Floatzel in Rain or Shell Smash Swift Swimmer) and finish off weakened things and with the Sand Power boost easily finish off weakened things. Swords Dance is ridiculous power. It can ohko physical defensive Skarmoy with Stone Edge in Sand and does massive damage to everything. Bulk Up gives surviveability and makes it harder to physically kill and it isn't weak to any common priority attacks (except Ice Shard which rarely ohko anyways except from Mamoswine and after Bulk Up do even worse and Landlos could actually survive Choice Band Mamoswine with some Defense after Bulk Up).
Terakion-Similar to Landlos with High speed and power and semi-unpredictability since it too can abuse Swords Dance/Rock Polish. Can't specially attack but it does heavy damage and Rock Polish allows it to kill the fast things while Swords Dance can allow it to kill the walls. In Sandstorm it gets a special defensive boost which can help allow it to survive some weaker special attacks and is again, really quite fast and strong.
Drought-Might as well since it'll probably become even stronger if Drizzle's gone and has some scary monsters of its own like Ulgamoth, Heatran, Charizard, Venasaur, Victreebel, Shiftry, Mebjuka, etc. The only reason it isn't used as much because it got the short end of the stick in weather inducer.
Latios-Insane speed and special attack and it's Draco Meteors hit like a truck and is still annoying to switch in on since little can survive double Meteors besides a steel or something incredibly special bulky and I mean incredible (who rarely have things like Slack off).
Honorable Mentions-Mew, Doryuzu, Deoxys-D, Ulgamoth (eh why not?)