[Ascovel]

Riven was indeed hard with deep complex puzzles, but from what I remember entirely fair.

Black Dahlia is another huge game with puzzles quite a bit in spirit of Riven.

The original Discworld from 1995 is also a game often mentioned when difficulty is discussed. It's a highly non-linear very long game with huge amounts of locations available at the same time and a lot of crazy puzzles. Though again, I'd say it's mostly in good fun.

But something filled with both absurd dead-ends as well as completely off-beat puzzles and unexpected deaths like Leisure Suit Larry 2 could be a good contender for - literally - the hardest adventure game ever. Though in a sense exactly opposite to what is currently considered good puzzle design and giving the player a fair chance.

Oh, and I heard there's no person alive who did beat Maupiti Island without a walkthrough... But it's a pretty obscure title.