The Microsoft screwup when the Gears of War 2 All Fronts Collection (retail value $20) was put up 2 weeks early on XBox.com for a few hours in the middle of the night for $10.
This was a tough call. I picked up the most games from Gamefly, and I almost bought a game per day during the Steam sale, but I voted for amazon since they had some amazing deals on new release games, plus the Tekken 6 bundle was killer.
I think Steam beat out Amazon this year, it's unfortunate that they probably don't have much of a chance since there aren't many pc gamers on CAG. They had some amazing cheap games for $5-$10, and insane bundles.
Gamefly gets my vote, Street Fighter IV, Ghostbusters, and Resident Evil for 13 dollars a pop plus local sales tax, BUT free shipping and in really good shape? YES!
Amazon's hands down.. I spent so much money.. ended up with Dragon's Age, L4d2, Uncharted 2 and Assassin's Creed 2. all for a hefty discount. I also had a bunch from the TRU B2G1 deal as well.