I can understand how you feel. AC2 wasn't that fun for me neither was KHds. Halfway through I felt like I was forcing myself to complete them. KHds it was after the tutorial missions. It wasn't the same with SFIV, RE:DC, or NSMBWii which I had fun with alomost everytime I popped them in.
If you think about it the games you enjoyed were those with multiplayer aspects in them. I have more fun in multiplayer games than in single player games. Just remember to have a balanced diet of multi and single player games and you should be fine.