Gen 4! Lucario, Staraptor, Gallade, Garchomp, Infernape, Roserade, Weavile, Leafeon, and Luxray are some of my favorites!
In terms of the content of the games, Gen 5. In terms of actual Pokemon, probably Gen 4. I also love all of those Pokemon mentioned above along with Floatzel and of course my all time favorite, Shaymin!