That would be Assassin's Creed 2 for me. The game JUST begins when you finish it (Collectibles, Bonus Missions, Assassin Contracts, Races etc.) It also has great great story.
Sounds like Fable
I can see how Fable would become addictive, many people complain it's too easy, and I can easily see why but I hope they know that I doubt Fable was intended to make you "max out" your abilities. Limit your abilities, go into Skill, some Strength and only Magic that could assist you in "assassination"-like formula.
Make your own classes, I think that's what Fable's all about
I'd say: CoH: Opposing Fronts Fallout 3 (including all the DLC) All the Total War games (endless hours of fun and working your mind, at least in my opinion) Bioshock 2 (I was so disappointed when it was over, I wanted to keep on playing even just maybe a free roam mode)
and the BEST for last: Red Dead Redemption Why: Amazing soundtrack, An actually legit Western game, Nonlinear gameplay, plenty of stuff to do, lots of guns to find, Superb Storyline (Though Mexico really threw off the fast pace of the game...), and even after I beat it (I hated the ending, just because it was so unfair to John Marston...), the Multiplayer kept me real, real busy for about a month or two after that.
