This probably won't be the most popular answer, but I'm partial to the piano.
The why is difficult to articulate, but I'll give it a try.
I like the versatility. It sounds good with a lot of musical genres; classical, pop, jazz, swing, Broadway, boogie woogie, etc.
88 keys allows you to (theoretically) play in seven octaves, giving the music a wider range of emotions, compared to some other instruments. An ukulele, for example, has only two octaves.
The tonality is fantastic. The instrument allows for a variety of musical textures, ranging from the very soft to the very loud, stacatto (detached) and legato (connected), among others.
You can sing along with the piano music, if that is your desire. That's hard to do with wind type.
You can play solo and other instruments are not required for it to sound good. Of course, other instruments will make the song/piece sound better. Being able to use two hands allows the simultaneous playing of a baseline and melody, something that other other stringed instruments which require bowing, plucking or strumming cannot offer (guitar might be the exception).