Wow, it's been a while! I started playing Doom in 96/97..? Not sure. Anyway, I got my start playing a pirated (!!!) copy of Doom II on a 486/33. At the time I did not realize this was actually illegal, I think a friend just installed it on my computer at some point without really telling me much about what it was. I discovered an icon called WinDoom (strangely, this was the beta build that was floating around on the net prior to Doom 95's release) and blasted away soon enough. I then moved on to Doom Shareware, which I located on some demo cd my dad (God bless him) got me for my birthday around the same time. Good times.
Later I got all the classic Doom series games legally through the id download store (and then later I got the Doom Collector's Edition as well). They were goddamn expensive too at that time (2003)! Running them on Win 9x was fine, but XP was a fucking nightmare using the Doom 95 launcher (which came with the 2003 download version for some reason). I ran into Doomsday/jDoom in 2003/4 and started using that instead. Nowadays, I use zDoom, mostly.
These are the Doom games I own (I've played and finished them all, except for Plutonia):
Ultimate Doom / Final Doom (id download store)
Doom Collector's Edition (the one with the DOOM3 preview disc)
id Anthology (this one actually included both DOS and Win95 versions of the games - weird).
Oh, and Doom 3, which I got on release date. It's a great game too, but in a totally different league. I really don't associate it with the classic Doom games.
I'm not sure why classic Doom still holds up so well. Perhaps it's got to do with the basic (yet solid) gameplay, both single- and multi-player (I use zDaemon for this, by the way - Skulltag won't run on my system). Also, there's a HUGE amount of community-made materials (level, sound, graphic mods, as well as the source ports which make the game playable on a wide range of systems) out there. A community that's been very active over the course of 15 years! Now that's impressive!
Shouts out to everyone still playing this now-ancient game! LONG LIVE DOOM!