Enom WildWest and BulkRegister are the main ones I was looking at
Who's the cheapest (.co.uk domains are important as I'm based in UK)
Who's got the best/worst support
Who offers the most/least ,features
Enom is decent, and relatively cheap. I've not had any problems with them. (If anyone's interested: I can set up 8.95 resellers from my account. I might have a 7.95 reseller still laying around somewhere...)
I have also used godaddy, without to much trouble. I don't like their website.
I have a few domains still with dotster(retail), but I'm moving them to enom because it's cheaper. I read some of their reseller docs, but wasn't willing to give up my right to take my domains elsewhere, or test other resellers if I wished.
And, just in case you've been hiding under a rock, DO NOT USE VERISIGN.
After contacing both Enom and WildWest with enquiries about their reseller packages Wild West definately came out on top in terms of the speed of their replies and the information they sent back was much more helpful and comprehensive.
If you just want to offer the basic domain registration tools to customers Wild West looks a good option, but for additioanl services like 'build your own site' services it looks to be at first glance that Wild West has more and higher add-on charges where Enom includes more features free.