Excellent subject.
Patagonian Toothfish....also known as Chilean Seabass one of the best tasting fishes out there is dwindling fast. It's still on the market although I am not sure of it's current status. I love it but refrain from buying it or ordering it.
The Monkfish is a common replacement for Chilean Seabass for it's plump texture and high oil content but those too are being fished out of control.
Southern California White Seabass - I participated in research for Atractosion Nobilis for 4.5 years in a hatchery catch and release program developed by OHREP and CA DFG and it turns out that protecting fish spawning areas really has caused the fish populations to bounce back so it works and there's more fish to be caught and turned into awesome steaks and tacos.
The Seafood Watch Guide really is a good guide for those if you seafood people. Stay away from overseas seafood if you can since there is unregulated use of Malachite Green which has been banned in the US for food uses so buy American if you see it
http://www.mbayaq.org/cr/seafoodwatch.asp