I hate to be cliché about it, but GameStop wins by far for:
1. Changing the expiration dates on coupons given as pre-order bonuses with games (i.e. the "$10 off on a future purchase"-type coupons). Coupons that were originally listed as expiring in 2012 or 2013 suddenly expired on Dec. 31, 2010 instead, with no notification or warning of the change.
2. A non-functioning website that often messed up orders, claiming that coupons weren't valid for items they clearly were valid for, and losing discounts in final checkout that had been applied earlier. Even when the previously mentioned pre-order bonus coupons were still valid, they frequently just didn't work when actually entered on the site, despite there being nothing listed in the terms that would explain them not working.
3. Rude and discourteous customer service reps. When I called once regarding one of those pre-order coupons that wasn't working, the rep refused to help me and treated me as if I had already used the coupon (I hadn't) and was trying to scam them (I wasn't -- I only wanted what had been promised to me). Completely unacceptable. (Back to the non-functioning website point, I tried that same coupon again two days later and it worked without issue that time.)
4. On a personal note, when I moved this past summer, GameStop refused to change the billing address on my pre-orders, and told me that I'd have to cancel them and place new ones instead. In the process, I lost discounts that I had on those pre-orders that I had gotten through earlier promotions which had since expired. Needless to say, I took my pre-orders elsewhere.