I've ordered from a few:
World Wide Corals: Great livestock, great selection. Prices can be high unless you order on a live sale. Pictures are fairly true to life, the coral you receive will be very close to the picture you saw of it.
Pacific East Aquaculture: Great livestock, even better service. Prices are moderate, there's usually some kind of a sale to bring them down. Not a great variety of choices for stickheads, but great variety in LPS/Softies/Non-Acro SPS. I feel like they don't get enough recognition for their quality.
TCK Corals: Quite a few interesting color morphs of acros, really good source for those. Pricing is reasonable, but not quite as much selection as some of the bigger shops.
Living Reef Orlando: I always forget about these guys when their live sales roll around (my email keeps shuffling their notices into spam), but they're another underappreciated vendor. Great livestock and great pricing, especially on live sales. Their selection isn't as robust as
WWC's, so there isn't quite as much competition - it can sometimes be easier to get what you want in their sales.
Salty Underground: Pictures and selection aren't great, but the actual livestock has been good and they have a very low threshold for free shipping. They also sell berghia, so I always like to pick up a coral or two when I need to bring in some of those heroic nudibranches.
Aqua SD: Pictures and livestock are good, but prices are quite high, even on live sales. Top-notch product though.
Unique Corals: Good quality, selection isn't always the best but they have a lot of info on the corals and their pricing is pretty good even when they aren't on sale.
Tidal Gardens: I don't remember if I've ordered from them, but they have great videos and write-ups about coral care, so I feel like they deserve a mention regardless.
FYI, it's always worth it to try to find an online vendor that's near you. Many of them offer cheaper shipping rates/lower free shipping thresholds for customers that are close to them. My LFS (Barrier Reef Aquariums) has free shipping at $350, or $200 if you're in-state.
