There's way too many but I'll throw a few in that have been mentioned and a few that haven't
1. Ms Pacman - on turbo mode. timeless classic that even kids nowadays will still play.
2. Ultima Online - sandbox game, first commercially successful graphical MMORPG although Meridian 59 might have technically beaten it to market. I'm still friends with people I met in game to this day. One of those friendships even got me a brief stint working at Sega testing video games back in the day before they quit consoles.
3. Super Mario 64 - similar experience to @sataneatscheese. saw a japanese demo in Electronics Boutique completely blew my mind. perfect game, imo one of the greatest ever made.
4. Dark Age of Camelot - mass scale pvp MMORPG. too long to explain in detail. if you played it you knew how bad ass this game was.
5. Metroid/Kid Icarus - 2 NES classics. played the snot out of both. great games and great music by Hip Tanaka.