Most instruments seem like they would be really interesting to be able to play, but if I had to choose one I would like to play most, out of all them, I think I would say a drum kit. I've tried them a few times before, without really knowing what I was doing and it was rather enjoyable.
A short, silly string instrument.
Lately I've been really into ukulele and mandolin. The latter would probably be cooler to be an expert at because it eases learning the violin.
Also: Reviving this seriously dead thread.