MSN did an article on this a couple of weeks ago, and they found that those Crest Whitestrips work the best since they have the highest concentration of the chemical that the dentists use to whiten teeth. There are some drawbacks however: cost, it only lasts a few months, and they only cover a couple of teeth past your eyeteeth on one side, and the same on the other (i.e. they will only whiten approx. half of your teeth on the top, and half on the bottom. This is simply due to the length of the strip).
The best thing of course is what the dentist uses. From what I read, the whitening effect lasts for years. The dentist's method using the same chemical the over the counter whiteners use, but the dentist uses a concentration of about 30 - 35%, while the most potent over the counter one (the Whitestrips) uses only about 11 - 15%.