Guitar (acoustic) because:

1) When I was little, I was a fan of the singing cowboys who all played it. Later, I discovered Country music, back when it was worth discovering, one of my first steps in annoying my parents.

2) My parents tried me to get me to learn on several instruments, but I wanted to try guitar, and they said no, they wanted me to learn a "real" instrument. Obviously, they hadn't heard of Segovia, of Django, or etc. I was a teenager, so naturally that made me want to play guitar all the more, and other instruments all the less.

3) When I was in college ('59 - '63) it was virtually mandatory.

4) I've always sung a lot, even when begged to stop, and it's the best instrument for accompanying singing that's even reasonably easy to transport (leaving out commercial air, of course).

Peter