Generation 5 by a long shot. I like the modern-ish look of the interface, the graphics are quite nice and it's very refreshing to see a region based on a location outside of Japan (in this case, New York City). I have a huge hate for Pokémon Diamond and Pearl (I can't fully explain it, but I really don't like it), so that kinda ruined Gen IV for me (I loved HGSS though, haha). I also much prefer the Pokémon of the fifth generation over those of the fourth - I was getting tired of all the unnecessary evolutions and baby Pokés Gen IV brought (I'm looking at you, Munchlax, Bonsly, and Magmortar~) and it was nice to see a Pokédex of completely new Pokémon in a completely new region.