The best game you have ever won

The witches enhanced

Skyrim Legendary Edition
Far Cry 3
Merto: Last Light (1 month before it was released)

Fallout New Vegas.
It completely sucked me in from the first time i started it (i actually played that one first before F3), have a bit more than 100 hours in it

so many good ones to choose from, I think the one with the most playtime that I won is Civ V, but most playtime obviously doesn't equate to best game I've won, just a bit indictive, i'd have to look through them all

and Portal Pack would be an awesome game to win, i agree :)

Civilization V

Sanctum 2