Well you can find a lot of quality companies like TuCows which is a Canada based ICANN domain registrar. There are a lot of companies and by visiting the ICANN website you would easily find the updated listings, categorized according to the countries of the firms.
I have experience with ResellerClub, ResellOne, NetEarthOne and Resell.biz.
Out of that selection, all of them use the same platform (the orderbox system) apart from ResellOne where the platform isn't so great (works best with their API). However, ResellOne's pricing is excellent and I have found their support perfectly satisfactory.
ResellerClub and netearthone are very similar. Both have very good support systems and similarly prompt support. However, netearthone probably win marginally due to lower pricing.
My experience is shortest with resell.biz, but from what I can see, they are the best/my favourite. They have excellent pricing, an excellent system, and are contactable via email offering good support. They are also part of the uk2 group which is a great company.
So, to choose the best registrar out of the ones I have experienced, I'd say resell.biz.