I have been playing adventures for 12 years and I can't tell which puzzle was the most frustrating. Many of them could reach the top, but can't remember them all.
However I never had much problems since I could soon find someone who could help me out or find a walkthru
The real problem though, that stuck me there for ages (no internet then, no walkthroughs in my magazines, no people to help), was Manhunter day one, when after the maze I reached the fair. Play again and again in case I missed something. I knew it was something about the dolls, and also tried various combinations to hit them.
By chance I found in my favourite comp-magazine a catalogue of game walkthroughs presented in past issues. So I went to an affiliating pc-shop that had an archive of these magazines and bought that issue.
The solution: hit dolls in a certain order, which you learn if you do something earlier, in a certain order
