It really depends on what service you are after I think. If you are in the business of registering 10000's of domains a year then the lowest cost is in your interest. If you just keep your own business domains in your account then the last few dollars dont really matter and then it's just a choice of which company to choose.

I don't have a lot of domains, just a few? and I bought them from my host.

we use melbournetit, onlinenic and resellerclub. But we prefer resellerclub they have nice interface and multi level control panel.