I really like the Pokemon introduced in Generation IV. I feel like they are all for the most part great additions and even the added evolutions of prior pokemon were amazing as can be (Gallade, Froslass, Dusknoir, Roserade, Porygon Z, etc.). I also felt that the plot was really good (Gen V definitely had the best and most convincing plot though), and it has some replay value to it. I also think I like it because it really challenges you out of all the Pokemon games I've played, because I know that getting to the Elite 4 and Cynthia is very painful.
Gen VI definitely is second or third on the list. I think with XY and OR/AS, it's bringing a lot of the old into the new, so to speak. It feels like they did a great job with the areas and bringing all the past Pokemon relevant again (not that they weren't, but the mega evolutions definitely bring light to some Pokemon). I still need to finish OR. I didn't get very far since school got in the way...