Games I've beaten that others haven't (due to difficulty, not boredom):
Comix Zone.
Bubsy the Bobcat.
Spiderman vs the Kingpin.
The 7th Guest. (Classic game.)
Also Tetris. Damn, the end guy was hard in that one.
The one game I couldn't beat in TWENTY YEARS (and not for want of trying) until I finally found how to do it on the Internet: Last Ninja 3 (C64).
The one game I wish I'd played until I'd discovered every secret, death move, hidden character and more: Eternal Champions CD. One of the best games that nobody's ever played but me.
And one game I wanted to beat but never could (that damn snake pit level!) - BattleToads.