Oh this is interesting, seeing a lot of the same ones pop up - funny how that works. I'll assume, since consoles are out of the running, that emulators don't count either?

Top 10, not necessarily in order (other than #1):
1) Without a doubt, Duke Nukem 3D 100%, hands down, #1 ever. The most gaming fun I've ever had in 30 years of gaming has been the countless, late-night Duke matches I ran in my old training lab. MAN those were fun nights! Not sure it would be as exciting nowadays but DAMN those were fun times! Jet Pack 4tw!
2. The entire Commander Keen series (remember those fun-filled Apogee days? Anyone? Anyone? Bueller?)
3. Wolfenstein 3D
4. Doom 1 through 3 - GOD MODE and CLIPPING with the Rocket Launcher, OMG anyone else remember the spatter sound of a monsters guts hitting the wall?
5. Team Fortress
Classic - yes, TFC not TF2! I still play regularly, me like-ah dah snipe-ah!
6. THE most addictive by far, you all know and possibly love (or hate), Ultima Online of course
7. Masscre and TM both mentioned Lode Runner, OMG I have to add it to my list too although I played with a cartridge in the back of my Commodore 64 so technically mine was a "console" version but WTH.
8. Half Life and Half Life 2
9. WoW
10. Rogue. Yes, Rogue. Hours and hours of paid, work-study in an empty computer lab with nobody to help so I ended up playing pure ASCII bliss. Thank you Office of Financial Aid & Scholarships Federal Work-Study program!
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