[Svencanz]

Easy question to answer:

Keyboards.

Because if you can play keys, you can play "anything". Given today's computers and software. I play flute, sax, guitar, bass, and keys to "household needs", and if I was given the right by God to immediately transmute into a pro on one of these instruments I'd easily choose piano (keys).
For instance, I hardly use the sax anymore because I find I can create "better sax solos" on the keyboard (using Reason) than I can in real life. If I do ballads I'd use the real sax, because of the authenticity and breathy sound.
Anyone can learn to play ac guitar for basic song-writing needs in two months, just get your rhythm sorted and pick up a few bar chords.
Piano, different story.

S