I would highly recommend NameCheap unless you're holding hundreds, then Fabulous is the best.
I've been very happy with moniker.com. I have 73 domains with them. They're inexpensive, I like their web interface, and I've had no trouble with them at all.
gandi.net is awesome
gandi.net
1and1.com
Name.com is the best. They have really quick and helpful customer support. Their website is incredibly clean and intuitive and they're not trying to sell you something on every page like GoDaddy does.
I bought a domain name from a guy in Brazil. We exchanged over 100 emails back and forth with the help of Google Translate and when it came time for the guy to transfer his domain, somehow Name.com had a woman on staff who spoke Brazilian Portugese who helped facilitate the transfer.
I'd never go back to Godaddy
Marco Arment made a blog post this week (http://www.marco.org/2011/04/14/why-is-it-so-hard-to-be-a-go...) that generated some chatter here related to domain registrars (http://news.ycombinator.com/item?id=2448061). The consensus seems to be that NearlyFreeSpeech.net is the one to go with.