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Gen 3 for the meta that had a large roster of viable mons in OU, a balance between offense and bulk, and didn't need 1000x clauses to stop gimmicks. Playing in the gen 3 meta has made it clear to me that the game is never going to be in that kind of balance again for a few reasons.
Power creep puts a huge burden on OU and sets the bar too high for all but 20 or so pokemon.
The special/physical move trait introduced in gen 4 was something that sounds great in theory, but the way it was implemented resulted in a hyper offense meta at first, and has only been counteracted by power creep.