During the Steam Sale I bought Borderlands 2 (half-off caused enough of my reliable friends to get it so that I can easily find someone to play with), the Binding of Isaac and Bastion (since both were super cheap Flash deals), and got Dead Island as a gift.
As for GotY... Well, I haven't played a ton, and I really wish I could have said XCOM or Guild Wars 2, but I'd probably have to go with Civ V's Gods and Kings expansion (that counts, right?) I haven't had as much time to play it as I'd like, but 4x games have grown close to my heart.
I think what I've really had the most fun with is Dead Island, though - it's a bit buggy and unpolished, but that's the perfect environment for three folks to run around smacking zombies for phat l00ts in.
(most time played award probably goes to Team Fortress 2, but that's a function of 'easy to drop in and out of')