I guess it depends on context. When I'm playing in jam sessions, it would have to be the dobro. It can flavor and enhance just about any kind of music, and there are already lots of guitars. For sitting around the living room picking out tunes by myself, it's gotta be the ukulele. They're easy to carry from room to room, they're also good for airplane travel and playing in hotel rooms. They are also very 'social' instruments, with 'ukulele societies' all over the place.
