Probably the original Discworld for me. Love that game for how much effort the devs put into it and it clearly shows they were huge Pratchett fans, but I found the game to be ridiculously hard.
Schizm 1 is also one of the toughest adventure games I've played. Riven is also quite hard, but once you grasp what is going on, it actually all clicks.
I'm not against using a walkthrough whatsoever, although hints are preferable as they allow you to get a different perspective or a small nudge as to what you're supposed to do, giving you the chance to figure it out yourself.