Amazon is the clear winner if you took advantage of their early promotions. GameStop had the best year round values, but I'm going to have to go with Amazon because I feel like I made out like a bandit due to some of their June-Sept promotions.
Definitely Amazon, for their additional trade-in incentives like the $75 console coupon and the 50% off game coupon. Unless they bring those deals back I doubt they'll be able to maintain it for another year, but for 2009 they were definitely on top.
I know it's canada only, but for a couple of months Rogers Video had too good to be true trade ins. (i.e., buy 3 games from them for $35 and trade them back in and get a $70 game)
They've fixed it up now, but they still get my vote.
Screw Amazon and their "there's a microscopic scratch on the spindle of this game so we won't take it, oh and that just so happens to stop you from being qualified for our promotion."
I'm gonna have to go with GameStop. As much as I dislike their corporate shenanigans and general employee buffoonery, you simply can't beat GS when it comes to trade-ins and pre-orders.
Its a tough call between amazon and gamestop, but my vote goes to gamestop. The trade-in promos have been awesome, and amazon takes a while to credit your account, plus its a shady company (nor-am) that's actually buying them from you.
Can I write-in Half.com? They helped me "trade in" several games for more than I paid for them. Plus I could use the money on hookers and blow instead of just more video games.