Yes, I'd have to say Riven was extremely hard; though that waffle puzzle wasn't that bad, except by the time I got to it, my brain was so fried from doing other puzzles that I gave up for a few days. I think coming back to it with a clean mind made it easier to solve.
I'd say that the hardest game for me to beat (not necessarily play) is Half Life. Most of the game is actually pretty easy to play through, but the final boss took boss fights to a ridiculous level. I think my brother and I spent a good 2 hours trying to figure out how to beat this thing. We eventually went online to see how.
Angry Video game nerd has tons of old console games that are ridiculously hard to play and beat; some of them are impossible to beat. Karate Kid comes to mind.
