You want hard? Try Myst. From what I hear, that's all puzzles! One reason I have no desire to play it.
Or here's a thought. Go over to
Adventure Gamers and look through their short reviews of the Top 100 Adventures according to them. It's a great list and you should be able to tell which of the games you'd like to play.
If you just want a wonderful game that costs next to nothing, go to GOG.com and get Zork Grand Inquisitor. It's on the list at #12.
Funny! And there are a few puzzles that should test you a bit.
Gil.