My first edition was 2nd Edition - pre option books, post player's handbooks - stole the core books from my brother and have held onto them ever since. As for my reasons . . . well I've got three reasons right here.
You will take my Giant Space Hamsters and my Lady of Pain and my Defilers from my cold, dead hands!
Now for an actual answer:
1) Innovation - Rules-wise 2nd edition was at best an iteration (look, we don't talk about THAC0) but 2nd edition saw an explosion in settings and D&D lore that continues to this day. Specialist wizards, bladesingers, bards as a functional character class, settings ranging from the weird to the baroque to the political: Birthright, Planescape, Dark Sun, Spelljammer, expanded Greyhawk, The Forgottem Realms (the TIME OF TROUBLES!), Ravenloft, heck, even Buck Rogers XXVC. If they could dream it, they did it and they did it BIG with a level or wild abandon and material support you don't see from single party publishers these days. Did they over-saturate the market? Yes! Did they publish themselves into financial ruin? Also yes! Did they give me Modrons and Tieflings and cannibal halflings? ALSO YES.
2) The Monstrous Manual - weird specific callout but the one-monster-per page, full write-up with ecology and culture? It's colored my expectations for every monstrous manual in any system I've read ever since. The closest thing I've seen since is Hackmaster's Hacklopedia of Beasts which came out twenty years later. Also the illustrations - special shout-out to the the Invisible Stalker - which leads into point three . . .
3) The Art. The Art. The Art. - 2nd edition had an insane lineup of heavy hitters artwise, and sometimes art styles would define entire gamelines. Dark Sun would not be the same without Brom, Planescape would not BE Planescape without DiTerlizzi and what would the Realms and Greyhawk be without the trinity of Elmore, Caldwell and Jeff Easly. 2nd Edition had art you'd sparypaint on the side of your van with pride.
