the AI and atmosphere of 5 is what killed it for me. 6 isnt a masterpiece in those regards either, but atleast the AI makes singleplayer more viable and its level atmosphere even at its worst is still better than most of 5's.
Currently playing: FTL - X-COM: EU - Hotline Miami
Tough one, but I'd have to give the edge to 5. If the controls for RE6 could be used with the older games, it would not be very far up the list honestly, but they do feel really fluid compared to the older games.