It depends on what you'd like to do with it. If you want to compose, I'd say keyboard is the way to go.
You can easily record all different voicings and instruments into a track. Even though most keyboards can't produce the sound the actual instruments make, it's still great to give people an idea of what you want to achieve with it.
Also I'd like to point out something a different user said:
"think it's the best instrument because, well, you can carry a guitar around (unlike a keyboard/piano), play with friends, sing while playing..."
You can do all these with most instruments, so I can't see why that would be a reason to pick an instrument.
However, it's depends a lot on what you want to do with it, for example, the number one instrument to rock with is a guitar. You can't really rock while playing on a keyboard. Well, some people try, but most of the times it just looks out of place and really odd