Uillean pipes. Utter cunts to play but just the most gorgeous tone to them. Phenomenally limited, in some senses, but in the hands of a
master, just heartbreaking. I suppose I'd say bagpipes in general, but for me it's the Uillean pipes that are the pinnacle.
Voice. I'm not saying this to be glib, but it
is an instrument, and a massively diverse and incredibly personal one. If I have to limit myself to a range, I'd say Basso Profundo, but they're a bit too rare to speak of properly.
Violin. As Nefeli says, it's the perfect instrument. It's also a complete beast. As some of you know, I play it. And, unlike the guitar, I can't ever imagine a time when I feel like there's a limit to how much you can learn on the thing. Preposterous, in terms of the simplicity of the idea and the complexity of the technique to playing it. I think that it's amazing that a complete genius like Heifetz would play things differently in his earlier career to how he did later - or rather, that that difference in playing is entirely audible.