Black or blue background. To get them attached and streak free, smooth a little bit of vegetable oil on the glass, lay down the background, and use a credit card or squeegee to remove any bubbles. This will hold the background in place, and prevent drips.
I have a marine reef background on my mantis tank--it's a weird sized tank, so there really aren't many choices. It doesn't look too bad, though I'd be happier with a solid color. The mantis prevent cleaners in there, so keeping the back panel clean is a pain, and doesn't get done when I'm busy (ie, most of the time).