I would agree that Priceline usually has the best deals. If you do go with them, as mentioned, check the betterbidding and/or biddingfortravel sites to see what people are paying and what hotels they're getting. If you're careful with locations and stars, you can get a pretty good idea of what you'll get.
If you're not into playing the Priceline game, look at the Orlando section in TravelZoo. Sometimes they come up with some excellent deals. Another place to look is BookIt -- I stayed at Regal Sun for $69 this spring using them. It also doesn't hurt to look at the traditional sites -- Expedia, Travelocity, etc. One final comment on BookIt -- they come up with some good deals, but you should read the fine print -- they have cancellation charges if you want to cancel or modify your ressie. Of course, if you do Priceline, you're really committed.