I like GoDaddy because they're cheap and realiable. Never had any problems in over 2 years I've used them.
I also like Mydomain.com a lot. You can register through their partner Namesdirect for $14.95 and then you get ton of free great features from Mydomain like email forwarding, subdomain creation, and DNS management all free. My last couple domains I used GoDaddy then just entered Mydomain.com DNS info so I could use their free features.
I have to agree with Scott. I have about 40 domains at 000domains.com, and every renewal that comes up I transfer domains to that registrar. It doesn't have the lowest prices, but sometimes it is worth paying more. I figure if I want to see this registrar survive for years to come, spending a couple extra dollars there is a small price to pay. Its very easy to transfer ownership of domains there too, which is great when your selling off some domains.
Btw, I have transferred domains from Register.com to 000Domains.com, and it takes just a few days. I never had any problems with transfers between them.