I use saltwaterfish.com as my main site. Then it is live aquaria.com, but usually for a harder to find piece from their WYSIWYG section.
I have also used reefs2go.com and for fish and inverts I'd stay away. They ship quite small and had several arrive dead. Great on refunding, but gets annoying. I use them primarily for their pods and an odd coral that'll show up that I can't find now and again.