Yeeaahhh, I'm going with Generation the First myself. They were too plain overall. I mean, don't get me wrong, stuff like Mewtwo and Charizard were great, but:
Pidgey.
Magikarp.
Nidoran.
Weedle.
Rattata.
-_-"
And I don't think every one in all the newer generations were super duper, either. Probopass, and need I say more?
I mean, if you were looking at 1stGen replacing 4thGen, you probably would not be impressed. I look at it like this:
Overall (and remember that word), if Generation A was released at the same time as generation B, would you still like most of the Pokemon in A more than most of the Pokemon in Generation B?
Though, it's all kinda paradoxical if you take into account that had we started with generation 3 or 2 or whatever, this following generations would probably be completely different than what we have now.
As for what I liked, the designs of Generations 2 and 3 were best (2 was kind of convincing to me somehow, and I liked the vibrant, uh, vibe 3 had), followed by 4, followed by 1, in terms of creativity and design.