Jason Fox has great stuff if you want to pay the premium, reefexotics.net is reasonably priced for nice frags. Those are the two online vendors I have dealt with and was very pleased with both of them.
Best would be hard to narrow down.. I have bought from alot of the guys here and a few that stand out are:
Rare Reef
Rocky Mountain Frags
Joe Knows Reefs
I have not dealt with Battlecorals yet, but they look like they have great stuff too!
Battlecorals
Rocky Mountain Frags
Joe knows reefs
Sexy Corals
I have been very happy with every purchase. You can't go wrong with any. I'm partial to battle corals though cause Adam is awesome to work with. Also, cherry corals has amazing stuff and Todd and Brett are also great guys.
I've had a couple of great transactions with Battlecorals. Adam is truly great to deal with and he's got great selection/prices. That's where I check first when I'm looking.
Also heard lots of good about several of the others listed; don't see any that I'd steer you away from.
Adam at battlecorals is amazing to deal with through out the whole process. Could never say anything against the guy. Perfect customer service packaging and a great guy.
Cherry Corals may not have a lot of the super-expensive-name-brand stuff, but what they have is amazing enough not to need the price-jacking label. Love the vendor, too. Unbelievable customer service.