I prefer to use Namecheap for brand new registrations because of their ease of use and reliability and I generally use Godaddy for their auctions.
It depends what you want, if you want a good clean system with the minimum of hassle and excellent services then go Namecheap.
If you want endless coupons then go with Godaddy but then you'll tend to get upsold endlessly on extras that others give for free, and while their site is packed with features it is hard to navigate; nothing being in easy reach especially for newcomers.
These two aren't the only registrars, just those I have the most experience of.