Probably the first gen. Nearly every team has a Chansey, Rhydon/Golem and Tauros, which is already half of a team.
Gen.2 is also the stall generation, with SkarmBliss and Umbreon running around.
Fifth Gen is the weather gen, for reasons that we all know.
I'll say that fourth gen is the least defined, because I haven't played third gen enough to get an idea of what it's like.