I'm probably not one to ask when it comes to difficult games since I avoid them like the plague
but I was just thinking the other day about Freddy Pharkas: Frontier Pharmacist, and how incredibly difficult and unforgiving it is. A lot of Sierra titles could probably be added to your list. Try on King's Quest III for size.
If you want the easiest game, any of the Humongous Entertainment games should fit the bill. Of course they are kids games.
Still kinda fun.
Sorry I didn't answer your post appropriately, just throwing those out there.