.....I think it depends....
if it's favorite instrument in general, I'm going to have to go with (what I think is) popular opinion and say piano b/c you can play any genre on piano and sound like it's plausible to hear it in such and such song.....conclusion: piano is a very versitile instrument
if it's what's my favorite instrument because of the way it sounds I'd prolly choose sax/trumpet or acoustic guitar but only in certain styles (i.e. my eyes will melt out of their sockets and my ears will shrivel up if you make hear country....><)
if it's which one looks more fun to play I'd say violin.....you can look really dramatic playing it, I think.
if it's which one people look sexy playing it it'd be cello or bass guitar.....don't worry I have my reasons
and so on and so forth....overall i'd say piano but I don't like picking....it's like picking a favorite song....