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Gliscor, Weavile, Garchomp and pretty much all pseudos (maybe except for goodra but he's not bad either), Gen III's starters, Slowbro, Machamp, Alakazam, Gardevoir, 75% of legendaries, Shuckle.
Those are just of the top of my head, I bet there's many, many more. Pokemon only tend to drop or increase in tiers when the meta or the pokemon itself changes (significant movepool increases, introduction of new types, generally affecting moves, new HA, etc) so most tend to stay good or bad unless something relevant to them changes.
