Ever since I first did some serious shopping on Amazon last year, it's been my number 1 preference for online shopping due to the ease of checking/modifying current or recently placed orders, and the ease of contacting customer service. Their customer service reps are usually very polite, helpful, and easily contactable (although sometimes I get reps who have accents I really can't decipher )!
Omg Amazon has saved me several times! Yea, sometimes the reps are pretty hard to understand and get through to, but I eventually get my issue resolved.
Another nomination for Amazon. I've only ever had "issues" a few times, so I used the email option to contact them. Each time I was responded to within a half hour with each problem already solved favorably.
Amazon. No question in my mind. When I had an issue with some denied trades from Nor-Am they credited me(promo credit, which is no good on Amazon's warehouse deals site though) for them with no issues.
When Nor-Scam sent the games back they were still mint/complete, so bravo for Amazon for doing right for their customers.
I'll go with the wildcard Best Buy Reward Zone program. I've had numerous problems with the program: two accounts, purchases not posting, etc. They have always taken care of business and thrown me some extra credit for having to wait.
I only say best customer service because the quality of some items arriving has sucked lately, and I've been getting more and more packages coming by use of creepy van/station wagon.