Most useful test tone frequencies?

Ninety-nine frequencies.  That's what I recommend.

A CD has room for 99 tracks.  You'll want frequencies for setting the crossovers, too, and you might want to put some sweeps on there as well.  A CD holds a ton of information, and even if you record every frequency that could be of any use at all there will still be room left on the CD.

For gains, two or three tones for each of the sub and the midbass.  You can make a few that are especially long in length to give you time to listen for clipping while you adjust the gains.  For crossover setting, you could record tones that are close to each other in frequency, sort of like a sweep but with separate tones, so that you can tell where your setting is at.

With all that room on the CD, end it all up with a few recorded songs that sound good for demo use.  Deep rumbling bass, hard hitting drums, silky vocals, whatever you want to hear that would demonstate a good sound system.  Then your CD will be the whole package.