Generation II, because of Crystal among other things (read below.) XD
Generation I was revolutionary, and the generation that introduced those critters into the world. :) However, after you beat the E4, you didn't really have much to do, except battle and trade with your friends, and of cource, catch em' all. XD
Generation II was also excellent. :D It showed Team Rocket trying to recover after being defeated by 'you' 3 (?) years ago, and continued from R/B/Y, which was nice. :) Also, after you beat the E4, you could travel back to Kanto, beat the Gyms there and VS. Red (or you), the silent protoganist we've come to love. :D Plus, most of the Pokemon's designs were great. :D
Generation III was alright. :\ I mean, I loved Dive, Team Aqua, Torchic/Combusken/Blaziken and some other stuff, but it didn't seem like the last generations, with little references. :| Plus, it intoduced Pokemon such as Relicanth, Huntail and Spinda. Yeah, I really dislike there designs (sorry :()
Generation IV? Gameplay wise, it was great! Wifi, quite a good story, underground etc. Plus, the graphics and heroes look good too. But.... the Pokemon really lack the creativity which made us love Generation I and II. I mean, Bidoof, Bibarel, Magmortar, Magnezone, Lickilicky, Lopunny, Bronzong. :\ I mean Magneton looked decent, but Magnezone? Three steel ball-things into a UFO? What the...?!?! Yeah, thats all I have to say...:P (Though there were some good ones, such as Cherrim, Piplup, Empoleon, Manaphy, Mantyke etc. And the Pokemon cries were quite good, too. :D)