The ones that stick out the most in my mind are Einhander from a PS1 demo disc and Sonic Adventure demo for Dreamcast.
Spent a lot of time in both of those, certainly more time than playing the full game for Einhander (since I never got the full game, and I think only ever saw it even once, at a convention decades later).
I still remember competing with my brothers in the Sonic Adventure demo to get fastest times, that was _mostly_ the same as the first level of the first game.