Rapps has some amazing fish. If he had what I wanted, I'd go with him first.
Blue Grass Aquatics also has nice cichlids.
I've heard great things about LiveAquaria, but I have never personally dealt with them.
I've had good experiences with Aquabid, particularly a seller named "Lotsoffish." I've gotten everything from rare cichlids to very nice crystal red shrimp. Only problem with him is that he only gets in certain fish at certain times. Can't really order anything special from him. All shipping is free from him. One time he shipped 4 boxes of fish and there were a couple of losses, but he sent out another box (free of charge) with a couple of extras thrown in as well. All in all had a ton of great experiences with him.