Best Game for a light gamer.

Alright, so here's the deal.

I play games. Casually. Mostly mutliplayer, almost never single player. They don't keep my interest long enough for me to keep playing. I'm not terrible at them, but I'm by no means good.

I've only ever beat one game, (Portal), and the only reason I beat that was because I had a friend walk me through the last parts of the game, otherwise I may not have ever beat it. I'm still working through Portal 2, and that may be my first game I beat.

I'm not looking for something that's child-like easy, I want something with average difficutly, but I need something that will keep my interest. So, any suggestions?

Here's a small list of games I've started, but never finished - if that helps.

1.)Batman Arkham City
2.)Sleeping Dogs
3.)GTA 4
4.)Saints Row the Third
5.)Half Life 2
6.)Fable (The orgininal)

Now, I've played many more games then this, however, these are the more recent ones that I've attempted, but they lost my interest, usually pretty quick into the game. Maybe I'm not a gamer at heart, and that's why I can't finish, but I'd sure like to try.

So, if you have a suggestion, I'd love to hear it, and cost is no object. I'll pay for whatever is good.