Hmmm... this is a tough one since I like them all, but right now Gen IV, since it basically got me back into playing Pokémon again. I was a bit burned by Hoenn and how boring I found the land to be, but after playing Diamond I started to like the series again. However right now Gen V is my favorite monster wise if only for all the new monsters that came out during it. What can I say, I love a good number of the monsters introduced in it.