Unfair: I Wanna Be the Guy isn't supposed to be remotely fair. It's part of the homage to ridiculous older games, taken up to 11. Cut-scenes are skippable though. They're Delicious Fruit, not apples.
Ninja Gaiden. Just Ninja Gaiden.
Most shoot-em ups, especially the Danmaku kind. Raiden for requiring superhuman reflexes. Touhou games for example, manage to cross the line. If you're good at them, they're a fair challenge, but some of the later bosses are just absurd.
Fair: It's been mentioned already, but Monster Hunter. Really the only things cheap in that series is the frustratingly limited storage space in the older ones, and the hitboxes on a couple of the bosses. (Plesioth, you know what you did)
A lot of Roguelikes (Nethack, ADOM, Elona) are absolutely brutal unless you know exactly what you're doing. Once you get past the vertical wall of the learning curve and stop doing stupid things, you have a much better chance of getting through.
Happy first post to Me.