He's not on RC anymore, but Bill Wann's 20,000 gallon tank would be in the same league. He has a thread on clay-boa. Except for the fact that I don't think I could afford one of the panes of acrylic on his display, let alone the rest of the setup it is pretty cool and he makes a lot of his own stuff which is always impressive.
Pretty cool guy, I had the chance to chat with him for a bit at Reef-a-palooza in Orlando.
nineball's is another jawdropper that comes to mind. Super meticulous build, I was stunned it blew a seam recently and the manufacturer was not very responsive. I think his insurance refused to insure him if he used another tank from the same builder...