Which one I play most?   sigh...  Eventailly I decided on mandolin, but the penny whilstle is so much easier to pick up (it is never out of tune - at least with itself) that it might win by any casual observation.  The mandolin is what I dream of playing though.  When I see myself on stage in front of a million adoring fans I'm playing the mandolin.   I wake up screaming a moment latter because I while I like being on stage, everything else about having a million adoring fans is something I hate.  (I'm not a socal person)
My life long dream has been to be a good piano player, but I'm not good enough to enjoy playing it like I do the mandolin.  So if I ever learn to properly play piano my answer will change.
I feel sorry for anyone limited to just one insterment though.  Or in my case limited to just 3 (4 if you count my voice, too bad I can't find the right note often)  Then again if I didn't have so many hobbies I might get something done on one of them.