Two very contrasting instruments come to mind. One is the piano/keyboard. As others have pointed out, there is really no comparison between it and anything else. Western music is pretty much based on it, the repertoire is huge, contains some of the very best things ever written, it is endlessly versatile.
I actually have an ancient keyboard, but it is not directly at hand at the moment. Piano was my first instrument, but I haven't played in ages. On the down aide, it is not readily portable.
And so, my other choice would be, for reasons I cannot quite articulate, the recorder. And no, not the soprano, with which kids torture music teachers in school. Alto and tenor all the way. Fairly cheap, easily portable, lovely, mellow sound, can play just about any tune. Plus, I like medieval and folks music.