Pokemon Gold/Silver for a good few reasons.
1)I liked the new Pokemon and the fact there were so many in the single game
2)Completing the Pokemon league was only half the game really; you could redo Kanto and then fight Red in Mount Silver.
3)The PokeGear was all pretty practical stuff.
4)Day/Night cycle which affected not only wild Pokemon, but also some trainers (the policemen near Goldenrod for example).
5)The week system and how it worked, with different events on each day.
6)Believe it or not, the music; to date, I feel they had the best stuff in the series.
7)The plot with the whole Team Rocket resurrection felt like a continuation of the previous game's story, aided by the fact that there were a lot of references/characters from that game. I like that kind of loose continuation.
8)The Gyms were all pretty good and the eighth gym felt suitably hard (Giovanni was a joke on Red/Blue).
9)It was the most revolutionary game in the series in my opinion, in terms of useful stuff.
10)They gave you a reason to hate your rival; he's an ass, so beating him feels worthwhile. Plus the fact he turns up so often and even in Kanto proper(only once I think).
I really have to say that since Gold/Silver, I have felt that the series has went downhill; I enjoyed Ruby/Sapphire and Fire Red/Leaf Green, although not as much and as soon as it went on to the DS, I've pretty much went off it. I hated Diamond/Pearl and Heart Gold/Soul Silver just wasn't as good because they changed too much stuff for the worse IMO; I hate the fact you HAVE to fight Ho-Oh/Lugia before you're allowed to challenge the Elite Four, which sucks because I can't be arsed grinding still my squad is able to handle Lugia long enough to capture it. There's far too much ancillary stuff I feel is unnecessary and, especially in the DS games, there is far too much grinding necessary to be able to do the Gyms.
