Mewtwo owned the RBY metagame, he was not too popular on the early simulators, but he was almost always used on Stadium and Link Battles (at least the ones I had, and I heard), Slowbro's Tobybro set was created specially with the intention of counter the spam of Mewtwos, and he still owned on GSC a lot (Celebi and Lugia did more, anyway), the reason why he wasn't used on simulators was because the RBY metagame was almost just played in-live, as the simulators start was a bit... obscure, and it was more competitive and a trully way to battle more for the end time of GSC.
Anyway, I think we're talking more about OU (OverUsed) than Ubers, as Ubers were more banned than not from GSC and nexties generations, and this's about the Most Dominant Pokémon in History, in order words, more than a just single generation. If ubers were available to being put on the rank, then Kyogre should be #1, Kyogre shapped the ubers-metagame a lot more than even Snorlax with the OU second generation.
About the place #7, I could rather nominate Chansey/Blissey or Suicune, Chansey was a must on RBY (a team of 6 Chansey even winned the first official japanese tourney), Blissey was maybe not the best Pokémon, but was an excelent option for some roles on OU, and was used a lot (maybe a bit more than deserved, but still), on ADV it's not even need to say how common and fearsome was the SkarmBliss core, and Blissey was still a top threat on DPPt (even if Chansey and Blissey are not take care as a single Pokémon, Blissey should still be on the Top 10). Suicune by other hand is the same definition of bulkyphazer that can check almost any offensive Pokémon on the game, Suicune + Raikou were an almost flawless core on GSC, on ADV, Suicune was maybe one of most dominant Pokémon ever with Calm Mind, and was still a top threat on DPPt.