The best instrument for composing music with, is the Human Brain.
Mozart and Beethoven did not use a piano, they 'saw' the music in their 'minds eye'.
This was why Beethoven wrote some of his finest pieces after going stone deaf.

If I am not right, please list the super compositions of any animal, other than man?
[teasing]

you know what this thread needs?

more cowbell

Being somewhat odd (and also not posessing a piano :P), I tend to compose a lot of Scottish stuff on guitar. I also find it quite easy to compose using a tin whistle, as it's very difficult to play 2 notes that don't sound nice together if you know what you're doing with it. Of course, I'm not the best of composers, so I'm probably not a great person to take advice from.

The didgeridoo is what the truly epic composers use. :P

Yeah, this is pretty much where it's at.

Piano is the obvious one. You can also just learn your theory and then all you need is a pen and paper and your good to go :P