GameStop, by far, for a multitude of reasons:
1. Their used games are nearly always in deplorable condition. Nearly every time that I've gone to buy a used game there, I've ended up having to pass on it because the disc has been beaten to hell. They have absolutely no quality control.
2. They continue to insist on putting stickers directly onto the artwork of games, which is a completely unacceptable practice.
3. Their employees are, for the most part, either rude, clueless, or both. For example: If I decide not to buy a game because it's not in a condition that I find acceptable, don't argue with me, don't be snide with me, and don't try to belittle me -- either apologize, just say "no problem" and move on, or don't say anything at all. And also, having an employee trick me into buying an Edge card by telling me that I'd get a discount which I didn't actually get (and then telling me that he couldn't give me a refund on the card afterward when we discovered he was wrong -- I still don't know whether he was really clueless enough to not know, or unscrupulous enough to lie to sell a membership) did not do anything to increase my willingness or desire to shop there.
4. Their online stock tracker is completely inaccurate. More than a few times now I've gone in to a store or called them because the website said that the store had an item that I was looking for, only to be told by the employee at the store that they hadn't actually had the item in stock for months.
5. If you're going to gut games (and while I don't agree with the practice in principle, I can understand the reason behind it) at least do so in such a way that the disc/manual/etc. remain in "new" condition. It's maddening to attempt to purchase a "new" game, only to find that the disc is scratched or the manual is torn because some lame employee couldn't be bothered to handle it with care. I shouldn't have to check the condition of a "new" game when I buy it, but in GameStop, I always do.
Well, when I buy games there, anyway -- which isn't often anymore.