Good question. There have been some good threads here with other recommendations so you may want to use the search feature to see if you can find them. I can't recall off the top of my head but seems like ncaditya recommended a decent one once.
I've only used Wild West Domains and chose to stick with them since they are part of GoDaddy. I just felt more comfortable going with a respected brand name. As far as the best deal, I'm sure there are better ones out there.
It's strange, though. I don't see many sites reselling domains (unless I'm just missing them). With as much money to be made and as many webmaster/web hosting related sites out there, I'm really surprised I don't read more about other people's experiences with domain reselling. I see a lot of domain and hosting affiliates but you can make much more as a reseller (long-term).
I think part of the reason is people are wowed by the $100 commission you can earn as a GoDaddy affiliate (for one sale) compared to a $3-$10 commission as a reseller (for multiple, recurring sales). But as a reseller you earn over and over and over again as that person continues to buy other products and renews their domains, hosting, etc.
Anyway, I'm babbling.