Moniker, for the following reasons:
I have worked with 50+ other domain registrars and none have come close to the level of service.
Of all the registrars I've used my "next best" experiences have been with:
NameCheap.com - Decent user interface. Pricing. You can get a helpful response, with some (small extra) effort. Thin staffing, i.e., took awhile to get a reply. Guess that was the downside of "cheap".
Enom - Decent user interface. Their support ticket system was a bit of a rub. They had some goofy legacy issues with their domain management system for awhile.
GoDaddy - User interface is decent, though a bit less than transparent at times. No 800# was a rub. (They may have one now.) Yuck! to the registration process that throws a bazillion "offers" at registrants. Support is helpful, at times some (small) nudging was needed and the process improved over time. Far more "corporate" than Moniker, so harder to wheel and deal, etc. Still, if I wasn't using Moniker I'd likely use GD.
Register.com - I used them for awhile. When they assigned account execs to larger accounts that worked like a charm, until the account execs started leaving one after the other - and I had to keep training a new one. A good account exec - and I had 1 or 2 - made all the difference, but RegCom was in other respects very corporate, i.e., answers were sometimes hard to come by.