Hmmm...ten games that left the biggest impression on me and are actually still fun for me to play...chronological order of when I played them...
Raid on Bungeling Bay (C64...one of Wil Wright's first games...nothing like The Sims)
M.U.L.E. (C64...my first multi-player [more than two people simultaneously] game)
Herzog Zwei (Genesis...my first RTS)
Mutant League Football (Genesis...Madden engine, but hilariously fun)
X-Com (PC...already mentioned several times)
Master of Magic (PC...already mentioned...and I agree...not equalled yet...)
Mechwarrior 2 (PC...already mentioned...very immersive when combined with Sidewinder)
Tekken 3 (PSOne...took a PSOne on naval deployment...we wore this out...)
Resident Evil 4 (GC...played through it multiple times)
Ratchet and Clank: Up Your Arsenal (PS2...good single-player...great multi-player...)
Honorable Mention (ie, ran out of room and I'm cheating): Archon (C64), Airborne Ranger (C64), Jumpman Jr. (C64), Questron (C64), Ultima 3 (C64), Kid Icarus (NES), Mike Tyson's Punch-Out !!(NES), Powerball (Genesis), Warsong (Genesis), Doom (PC), Master of Orion (PC), X-Wing (PC), Full Throttle (PC), Duke Nukem 3D (PC), Heroes of Might and Magic 2 (PC), Warcraft 2 (PC), Wing Commander: Prophecy (PC), Freespace 2 (PC), Baldur's Gate (PC), Fallout (PC), Diablo 2 (PC), Unreal Tournament (PC), Crash Team Racing (PSOne), Twisted Metal (PSOne), Star Wars: Rogue Leader (GC), Eternal Darkness (GC), Metroid Prime 2 (GC), Zelda: Wind Waker (GC), Mario Karts: Double Dash (GC), Ace Combat 5 (PS2), God of War (PS2), Guitar Hero series (PS2), Karaoke Revolution series (PS2), Buzz! series (PS2), Gears of War (360), Force Unleashed (360) and a few more post-1999 PC games but I've abused the thread enough.