deepdiscountdvd is definitely the best.
www.dvdplanet.com currently has great deals on all criterion dvds, even cheaper than deepdiscountdvd. They have free shipping on orders over $30 (or $25)? And if you register a new account, you get a $3 off coupon code.
So right now, I use dvdplanet for criterions, and ddd for just about everything else. amazon.com sellers once in a while have something cheaper than ddd, but you have to factor in shipping ($2.50 for a single dvd) and look into the sellers feedback and make sure they dont sell junk.
here's dvdplanet's criterion link:
http://www.dvdplanet.com/criterion/criterion.asp