There is one thing I really can't bear, I think it's the Yamaha DX7 synthesiser - well, not the synth itself, more a couple of the sounds; firstly, that kind of wheedling 'glass harmonica' sound as heard on Tina Turner's "What's Love Got To Do With It", and secondly that kind of dreamy, teary, chiming, smug-sounding (copyright Bill Bailey) almost-piano found in the moody intro of every American 'love-song' in the 80's. Yes I mean you Whitney Houston.
I suppose the human voice could be called an instrument too, in which case there are plenty of those which really grate on me, but overall I can't really think of an instrument I totally hate. I even like the bagpipes