If I had to? Undoubtedly my clarinet. Now, maybe I'm biased as I've been on a music kick lately, but I don't think it's something I could live without. There is something so incredible about being able to play that marrying the thing which allows me to make music that it's probably about as close to wedded bliss as one can get without, y'know, actually being married to a person.
Come to think of it, (and any fellow band geeks will understand) there are actually a lot of parallels between one's instrument and one's love. You take care of it. You fix it. You're protective of it. It can become an extension of your very being. You can get annoyed with it, and you'll proably forgive it. You can leave it and come back to it. It requires a little effort to be harmonius.
Then again, being a young and inexperienced person, maybe I'm wrong :D But... that's my answer.