I use Internet.BS the cheapest registrar on the net I believe and I also get great customer service from them.
I like GoDaddy for domains but hate their hosting. I find 1&1 offer good value on domains if you're using their hosting but customer service can stink at times. I've also used NameCheap and they're pretty good too.
if you have large number of domains, PRICE is main factor. I am not crazy about my registrar, but they give me serious discount over their published rates.