Voice gets my vote....
It's portable, can be played soft or loud, and doesn't require a case that blocks the hallway. Unless you're Tibetan, of course, it only provides one note at a time, but used well and in good combination, the possibilities are endless.
Other than that, mandolin is good, especially in tight quarters. But if I could only have one instrument in a session, it would probably be a guitar, for the range, flexibility and bashability.