Guitar. This coming from a keys-player. Guitar is so damn fiddly to get right. I don't know how guitarist play elaborate solos. I can't even get the intro to Back In Black right. Also the theory is difficult, you have to have an understanding of keyboard scales, it's all just very fiddly and particular.
@Briggett I had instructor drop as student by because of my instances on wanting to learn how to play cords and hold the melody using my left hand being right handed