Most years I would say Amazon without question; however this year I felt Amazon continually said "phewy" to PC gamers for 11 months of the year. In particular Battlefield 3 not getting the same amount of a discount as the console version [$20 promo vs $10 savings] made me very angry. I don't remember Skyrim getting an Amazon discount either, but I could be wrong on that. Additionally, the very few titles I actually wanted to purchase at release received promo credit.
At the same time, 2011 was a really weird year for games. It was literally 10 months of dead followed by a ballistic 3 months - maybe it wasn't all their fault.
Amazon's Black Friday video game deals were atrocious. This is the first time in at least 4 years that I haven't purchased anything from Amazon on BF.
December Amazon was being really competitive with their PCDD deals.
During Amazon's Holiday Sales I picked up
DX, Shogun II [with promo coupon] and Witcher 2. All prices beat Steam.
I also picked up Zelda for $39.99 after Christmas [which I still count as the holiday season]
Steam is a company I don't really like to bring up. I don't really like the way they do the product, however with boxes essentially being flimsy crap now rather than being a visual centerpiece, I don't care about the boxes often anymore.
Steam had, for me, unexpected Black Friday Sales, and I ended up purchasing three games I had zero intention of purchasing:
Portal 2
Fallout: New Vegas
Dungeon Defenders + DLC [count is basically as one]
Steam's anticipated Winter Sale led me to purchase:
Batman: AA
Shogun 2 DLC Xpac
Plants vs Zombies
I also saw other games on Steam that I might have purchased had I not developed a very large backlog, but probably not.
Because Amazon's titles were newer, even though they weren't as steep of deals as Steam, beat some of the prices on Steam for the winter sale items, and their meager discount on Battlefield 3 I think Amazon still slightly wins for me this year.
Winner: Amazon