Hardest is definitely Schizm. It's so hard I don't even understand the walkthrough.
Easiest is Undercover: Operation Wintersun, it's also the most boring game I've played.
Dracula 3: Path of the Dragon I also had a lot of trouble with. It started out great with lots of really cool very doable puzzles but somewehre at the start of the (extremely long) third act they suddenly started pushing up the difficulty to an impossible level. The last couple of locations I played with a walkthoughprintout next to me.
