Most definitely toysrus.com
At least with GameStop/EB you know what you're getting into (ie: used games may come without manual/case). TRU.com is always full of surprises - be it cancelling orders that were previously listed as 'Inventory found in warehouse', items not showing up on your invoice, or the major problems they have integrating with Google Checkout. Not to mention continuing to leave items for sale that they know are out of stock.
Also, if you try to cancel an order, you'll run into a few more problems. Firstly, you have to cancel by phone. This means navigating through an annoying menu system, and then waiting on hold for up to an hour. Once you actually speak to someone, rather than cancel the order on the spot, they have to 'submit a request to cancel' the order - 'usually cancelled within 24 hours of the request.' In the meantime, if your order ships, you'll have to ship it back to them to get a refund.
But I'm not bitter.