Hmmmmmmmm..... This is a tough one to answer.
Seeing a lot of old games I used to play.
I guess mine would be the Sonic on the old Sega System, Super Mario on N64 and some old PS1 games like Tomb Raider 1/2. Digimon World is also up there for me; first game I got on PS1 and spent hundreds on hours on it - shame that not many people know about it.