Generation II, pretty easily. I love G1 and give it a ton of credit for starting everything off, but there's no matching G2 IMO. I started on G1, but I remember all the speculation and rumors about G2 before it came out; it was pretty exciting, actually, to see Pokémon like Snubbull, Marill, and Donphan in the first movie (not to mention Togepi in the anime) and to try to figure out what other new Pokémon there were, and everyone got really excited when we realized that not only was there gonna be a new region with 100 new Pokémon, but there were also baby Pokémon and new evolutions, and two new types! Not to mention the day/night system, berries, and the ability to go back to Kanto after you finished the game. The Johto Pokémon were all great and they melded nicely with the G1 Pokémon, and let's not forget that it also includes the excellent Pokémon Stadium 2, which improved upon the already-great formula of the first Stadium game.
G3 was a huge letdown for me at the time it was released...while I've come to appreciate it more over the years, I was always hugely disappointed by the fact that you could only visit one region in the game and that you couldn't even catch all the Pokémon--you couldn't even trade from previous generations, and a lot of my favorites were left in the dust. They also got rid of the day/night system, and while I thought there were some improvements to the game, I found that overall the changes made for a less enjoyable experience than G2 for me, although FRLG was a big improvement to the generation, as you could now use most Pokémon again. I also didn't really like many of the new Pokémon, and none of them really even come close to my favorites from the first two generations. I still enjoy my G3 games, but I'd much rather be playing Crystal or even Yellow than Emerald. I haven't played G4, so I won't comment on that, except to say that I don't like many of the new Pokémon designs at all.