I have used Netflix, Blockbuster Online, Greencine, and Nicheflix for online DVD rentals.
The best is Netflix. They are the most dependable: fastest turnaround, largest selection, and best Web site. They have the best blogger-fans, too :). The Netflix blog is written by the excellent James Rocchi.
Blockbuster Online would be my second choice, but they always have availability problems. I am testing BBo for comparison. No more than two-thirds of my queue has ever been "available now". I can get mainstream hits from them, but the four art titles in the top slots of my queue have been "long wait" since January. I never know what to expect from them; they never send titles in queue order. The closest distribution center is Charlotte, NC, about 90 miles away, making delivery time about two days or more.
Nicheflix is really good for titles that are unavailable on DVD in the United States. They specialize in multi-region discs, which require a special DVD player. Their distribution center is in Indiana, about 500 miles away, so delivery time is three days or more. They don't ship in queue order, because of availability issues. I like their Web site, but anything other than title searching requires using the "advanced search" feature, which is inconvenient. I can't be sure the discs will be in English or have English subtitles, so I have to check the Web site carefully before ordering.
Greencine has availability problems. Their only advantage is that they offer out-of-print titles on DVD. Their only distribution center is in California, 3000 miles away, so delivery time is 4 or 5 days. I like their Web site, because in addition to personal recommendations and member lists (like Netflix), they have Movie Primers and Featured Top Lists. They have a cool blog.
If you are looking for a title that has been released on VHS, but never on DVD, Facets Multimedia has an online rental option. They also sell and rent VHS tapes, DVDs, and books online. I can't evaluate their online DVD rental service, but their online VHS service is pretty good. They require you to email or call, but I've experienced excellent customer service.