Depends on your criteria. I like fabulous and dynadot and enom for different reasons:
Fabulous - free private registratons, able to search portfolio by nameserver, great expiry control, fine personal support.
Enom - great API for your registration/hosting business, and supported by WHMCS out of the box, fine personal support.
Dynadot - good pricing on some of the odd extensions that everybody else has high prices for, some interesting domaining functions
(All of the above have good pricing on the usual domains.)
I DO NOT LIKE:
GoDaddy (absurd couponing/discounting games that must be played to get a decent value, aggressive cross-selling makes navigation a pain, weakness in applet makes management unnecessarily difficult if you are accepting pushes of domains; soft-core porn used wherever possible)
DomainSite (prone to errors and not following thru on commitments, weak applet re: subaccounts),
OnlineNIC (horrible interface, weak support, and have lost domains there; limited ability to communicate in correct Engilsh)
1and1 (perverse accounts/packages scheme, and dim support personnel, and doesn't handle ws domain transfers properly)
NetworkSolutions (very high prices)
