Hmm I have a few but only in order of me remembering them:
*StarCraft - This and the expansion. By far the greatest RTS.
*Ultimas 2,4,5, and 6. - Great fun on my old Apple IIe.
*Half-Life - Nuff said.
*Samurai Showdown - Neo Geo game but passed countless hours of college time with my buddies.
*Dragon Warrior and Legend of Zelda - Two of my favorite NES RPGs.
*Duke Nukem 3D - When online gaming was born on the TEN network. Back when you paid $10/month for a chatroom and game servers.
*Doom - duh. This game was SCARY when I first played it. I downloaded it onto 5, 3.5" disks back then. It didn't take long before the entire Computer Science lab was infected.
*Quake - DM and CTF. Don't ya miss that grappling hook?
*Street Fighter II - I loved those nights of being in the 'zone'. 7 or 8 quarters lined up on the machine all waiting to knock me and Sagat off the machine.
*Metroid(s) - Have you played one you didn't like?
*Mechwarrior 2: Mercenaries - Good times.
*King's Quest 5 - What I bought to break in the new IBM/PS2 286.
I'm sure there's a few more but these are all great games. To me any ways. They all bring back wonderful memories I've had from growing up to today.