Several games.

Theme Hospital (PS1) - weird game, a bit confusing, and the fact that some text aren't displayed properly doesn't make it easier.
Roll Away (PS1) - a puzzle game where the main character is a - rubber ball. It's not that hard at first, but as you progress, the puzzles become pretty tricky.
Chu Chu Rocket (DC) - another game where you have to use the old noodle. Aside from logical puzzles, you can compete against other players, which I plan on doing.
Kao the Kangaroo (DC) - a mediocre platform game that doesn't bring anything new and original, plus the save/load state system doesn't work in it. I abandoned it pretty quickly.
E.O.S. Exhibition of Speed (DC) - a decent racing game, but the controls should've been better. Steering isn't easy, and navigation in the menus is badly executed.