NAMECHEAP.
Tried 4 other popular ones, and once I reg'd only 1 name with NC, decided I'd never go back to anyone else, and will transfer all my names to NC eventually.
Also have a good friend in town who's one of the domaining pioneers who got into it in the '90's when you could still buy prime names - which he did; he has quite a few thousand names, and uses ONLY Namecheap. Now I can see why.
Services are wonderful, customer support is practically instantaneous and perfectly helpful; problems - the very, very few times they occur - are solved efficiently and totally satisfactorily.