Fry's is well known, as was compusa although they got in legal troubles I havent heard if they've really improved. Tiger Direct is supposed to be pretty bad.
Seagate I believe holds the current record for rebate suckage.
In all my years of rebating (and I use 'jennifer johnp's method of logging the amount, date, due date, 800# and email to complain, and copies of all the materials sent for anything over ten bucks) I've had one crappy digital camera company go belly up and not pay a rebate ($89 off $89) at all, HP tried to weasel out of a rebate mistake where they issued 3 different rebates on a laptop, none of which excluded other rebates or offers - they paid, Seagate tried to pretend nothing happened but when I called, they paid, and t-mobile sent me an old rebate form with a new phone, then although I included a letter explaining what they did, rejected it for being an old rebate. When I called, they sent me the new rebate form. Which was identical to the original form, except for the expiration date, and went to exactly the same rebate processor.
Rebates processed by the company in "New Rochelle, NY" have also been frequently maligned.
Best ones are Costco and Staples. You can do em online and they pay fast.