I've been playing FireRed lately. I've had both it and LeafGreen for YEARS, but I'd never actually played them much - I think I just migrated the starters over to Gen 4 kind of thing. Gotta say, I think it's my favourite Pokemon game. In general, I love Gen III - they offer the fastest gameplay in the series (seriously, the battles whip right along and everything feels crisp and great), the graphics are clean and fantastic (best graphics in the series, I'd argue)... It's great. And between the spot-on remakes of RBY and the most complete-feeling Pokemon game to date in Emerald, there's a lot of greatness to be found. The only thing I'd knock it for is the stupid Natures system (which is where Pokemon games started getting way too complicated), some boring starters in RSE and a generally boring selection of Pokemon in the early stages of RSE. FR/LG is probably the best Pokemon game ever made, with Emerald and Crystal trailing slightly. I really don't like Gen 4 or 5 - the graphics look really jaggy and bad, and the games just play waaaaay too slow... It's like trying to run DOOM on a Dell - just chugs along to the point of frustration. Plus they've added all kinds of shit that makes it all way too complicated and just... Not fun to play.
But yeah. What Gen is your favourite?