I'm with Amazon, mainly because as a retailer they have always been really, really reliable and helpful.
I've rented mainly tv series with them so far and they've always come in order and very promptly. The turnaround is usually 2/3 days max. You also get 10% off dvRAB that you buy with them.
They may not be the best value in terms of number of dvRAB you recieve but they are reliable.
Last month I cancelled my account as I was moving house, and was told to return my latest DVD in 15 days. I forgot when it was due and ended up being charged for the dvd. When I emailed them saying that I would return the dvd and, as I was in my new house, wanted to re-start my account, they refunded the money immediately. And they were very polite and helpful doing it.