Hmm.. This is a tough one. Lessee... Not in order, and I'm going back (mostly) to classics. Some of them are relatively new compared to other entries in the list though.
- Wonder Boy III: The Dragon's Trap (Sega Master System)
- Quake 1 (primarily for QW: Team Fortress, back before speed-jumping became all the rage.)
- Descent 1
- NetMech (the free Multiplayer addon of MechWarrior 2)
- Doom
- Commander Keen (specifically #4 which was definitely my favourite of the series)
- GTA: San Andreas
- The Elder Scrolls 3: Morrowind
- The Quest for Glory Series
- Monkey Island 1/2.
Cutting it down to just 10 is rather difficult! I spent a lot of time playing through the Wing Commander series' as well, Ultima 8 (first of the Ultima games I had exposure to while they were current) and all sorts.
Ooh crap, and Return to Zork! Civilisation 1!
So many good games.