Depends on the type of tank, location, whats in it, etc.
I'd love to have a nice room divider tank, clear on both sides. Maybe starfire glass to boot.
If its just up against a wall or an in-wall tank, then I like backgrounds. For a tank with lots of scaping, plants, etc. I like black. Makes any color in the tank stand out more, and doesnt distract the viewer from the scaping. If there is minimal scaping then one of those textured rock 3D backgrounds would be great, or just the vinyl printed ones.
I did black vinyl on one tank, but could not get all the bubbles out / they came back later and now doesnt look so good. So the last tank I did I painted it black, just with latex pain. Looks much better, nice perfect black with no chance of bubbles, etc. It did take several coats though.