I couldn't give you a top 5 without a lot of thought, I've been gaming 10+ years and can't quite agree w/ the most recent FPS games being top of all time...
One I can definitely agree with is Half-Life. I doubt any game has sold more, or been played more years by it's loyal fans.
Next up would be Red Alert, maybe...Blizzard games are strong candidates too.
One I don't think anyone has mentioned - SCORCHED EARTH! I loved that game. I still get a kick out of it.
Tribes is also awesome - I missed out on the original but was a huge fan of Tribes 2. Everyone talks about vehicle support, that's where it started and it still holds up well.
My first system was a 386 SX 16Mhz like someone else
40MB HDD doublespaced to 80!!! 1MB RAM, 3.5" and 5.25" floppy drives