Dunno if they're the hardest, but they're the ones I find most obvious / most annoying when they're 'wrong'.
Piano. While you can tell that the sound you're hearing is piano, the percussive nature of the instrument (hammers on strings) often does not come through. Also hand percussion instruments tend to not have the full range of sound you get when you hear them played live.
harmonica, no seriously, it can sound off, even on some good systems.
I agree with @Merrick on piano but also most other stringed instruments, bowed or plucked.
It's hard to get the texture right on oboes; sometimes bassoons too.
Separate names with a comma.