I'm torn. I think Gen I and Gen II are about tied for first for me.
Gen I: Starting off fresh and new. "Wow, check that out! Ooh, what happens when I do this???" I was instantly charmed and have been for a decade and a half now. The sheer newness of it all was its biggest draw, I think.
Gen II: "You mean I can evolve my SEADRA???" I was hooked. I know a lot of people slam Gen II for only adding 100 new pokemon, but you've got to figure, it was the first expeansion. They had no idea how many new pokemon kids could soak up at once, so they played careful. I have to say, though, for only adding 100 they really did well at creating pokemon that would go on to become fan favorites. Plus, the thrill of defeating the Elite Four only to find I could go through Kanto again was amazing. (And shinies, YAY!)
Gen III: I think Gen III gets the short end of the stick. Part of the problem was probably the technology jump. Many, MANY fans were horribly disappointed when they found out they couldn't transfer over their Lv 100 shiny, max-stat Charizard. I have to say I was one of them. (Not Charizard, though, Espeon) Setting off through Hoenn starting in a bad mood, you're not going to enjoy it as much. Some of the pokemon were more kooky than endearing, but there were a couple gems. (Like getting a Ralts super early in the game, only to realize what a bugger it was to train at that level) If you base Hoenn off itself, and don't try to compare it to Gen II or try to see it as a failed continuation, but a fresh start, it's actually nice. I loved to OW weather, personally. That was one of my favorite features.
Gen IV: Eh. Not sure what to say about this one. They managed to keep some level of connectivity, albiet limited, so fans weren't TOO unhappy. (although having to defeat the elite four first was unhappiness) Some much-needed evolutions were finally given (Tangela and Lickitung) and some very unneeded evolutions were also given. (Duskull) Some of the pokemon are downright cute (Piplup) and some.... not so much. (Skuntank) Overall, a mixed bunch, but not horrible. I liked the world, although I really do wish at this point they'd stop having the 'mandatory one island gym town' thing. The baddies were badder than ever, this time out to destroy the whole universe. Cyrus was..... interesting. Maybe the first step toward the maturation of the audience like we're seeing in Gen V previews. Compared to him, Giovanni is not only a calm, rational person, but his generic slavery seems downright fatherly as compared to the destruction of the entire universe.
Gen V: ??? We'll see. So far it's looking good. Only time will tell.
EDIT: Haha, that took so long to type I got logged out.