I'd say the Renaissance lute is the most difficult instrument. Right hand thumb inside position takes ages to master; sitting even slightly wrong can give you terrible back ache for days (the lute is slippy and bowl backed); it is VERY easy to get a horrible sound out of it and takes ages to produce a lovely, full, round, fat tone; you have to get your right hand right inside and between 6 and 10 courses of closely spaced strings (depending on your period of lute); the music for the instrument tends to be comlex; and the left hand stretches can be inhuman.

Still, it is a beautiful instrument - just to look at, let alone to play.