Gabriel Knight 2 might not be the hardest, but it was one of the very first games i've played, being still quite inexperienced adventurer
I remember it took me a month to finish it.
Timelapse
is notorious for being one of the hardest Myst-clones around. I can't say it for sure since i was stuck right at the beginning
Black Dahlia is also extremely hard. LucasArts titles are generally easier to older Sierra games simply because of the fact that majority of Sierra games utilized interface without "smart cursor", and also featured "dead-ends", deaths, and even "be at the right place at the right time". If you can finish Conquest of the Longbow, you're a true hero.
