I personally use
Namecheap... They're about the same price as GoDaddy but you don't have to deal with all the obnoxious advertising trying to sell you stuff you don't need (which is what I absolutely detest about GoDaddy).
I'd stay away from Register.com at all costs. They're extremely expensive AND when your domain expries, they call you on the phone daily to hassle you even if you already renewed or transfered your domain elsewhere.