I was with eNom for a while but then just dropped domains completely because of eNom's no-cancellation policy. I'd get an order for a domain, find that it's fraud and then be stuck with the domain for a year, or more.
I currently have accounts with ResellerClub, Resell.biz, and Enom. I've almost never needed to contact support at either of the three, things tend to just work as expected. There was a short DNS outage at LogicBoxes registrars a couple years back, though it only lasted for a few hours.
We use PlanetDomain/TPPWholesale and recently added ResellerClub as part of a migration to WHMCS. I always had issues with the PlanetDomain front end and we never got it to work. I've tried other stand alone front ends as well and integration is a pain. Now that we are implementing WHMCS, domain registration works better (but of course a whole set of other problems and issues). We are happy to get away from a manual ticket process (which WHMCS does well with both TPP and ResellerClub). As far as service, TPP has been excellent as far as domain related issues. We haven't yet had a domain related support request with ResellerClub so I can't say anything about that. Though their support for WHMCS integration is fine.
If you are using WHMCS, it is a little confusing as there are separate sales channels with ResellerClub. We opened an account and then the sales person couldn't really provide the better pricing on the lower tier so we opened a second WHMCS specific RC account. I wish they would resolve that internal disconnect.
Ultimately, we get a good mix of prices between the two. And both have offerings that are unique.
We use NetEarth One and on the whole happy with the service although we have had a few issues over the past few years with domains not registering they just seem to be frozen but then other orders go through fine.
We use eNom and can only say good things about them! The whole system from their site, control panel, ordering system, all the way to the API integration into WHMCS and other similar products works flawlessly. However, on the flip side the prices are a little higher then some of the competition we have used in the past.
If you wanted a more tricky CP to deal with but I believe lower prices (haven't used them in awhile) then NEO/Net Earth One is great to deal with! Their customer support is superb and always willing to help and throw in some discounts/promotions.