I'm sure people will flame the hell out of me for this (especially since I'm the first one) but: GameStop. No, seriously. Their B2G2F on used PS2/GC games, GBA clearance and other random sales (50% off PS2 games, $20 120GB refurb 360 HDD, the B2G2F on PS2 games stacking glitch) sucked up a lot of my money.
GoHastings has been trash since they revamped their website/pricing system, Amazon Warehouse deals is too random, and GameFly only had a minimal selection of quality titles. I've got so many GS receipts this year.