While I've probably played a few C64 expansions, the first console titles I recall would be Sonic & Knuckles and Street Fighter Turbo and Championship Edition. The first pc expansion I bought would be Mechwarrior 2 Ghost Bear's Legacy. Mercenaries was also great but I received that for my birthday. The first pc expansions I'd played were friend's copies for Wing Commander and similar such ilk.
There are definitely some great titles in the thread and really shows the potential of what gaming could deliver, as well as potential downfalls that currently plague gaming these days. The disparity between new quality content for a fair price and cut content pre-release(which I know varies and is more conspiratorial) to be sold later as expansion passes, not to mention the swathes of overpriced cosmetics and season passes is absurd.
Looking at the Everquest expansion also got me. I sincerely miss the days when great artists like Keith Parkinson graced covers and adverts. Handsome warriors, lovely maidens, creepy monsters and villains. Hell my gf noticed the theme of Leon saving Ashley in RE4 which I've been playing lately. I miss saving the princess ala Mario. Also playing as the heroin is cool too. I guess my main gripe is that when I was younger from childhood to teen and adult, what I imagined gaming would become didn't quite pan out. The gfx are there but the art(direction, style), stories(concepts, writing), and gameplay(evolution, innovation) just have not wowed me. I'd almost say the success of the gaming industry has been a double edged sword, attracting entities that would rather stifle innovation for monetary gain rather than deliver experiences that are more ambitious and greater than what's come before, whilst kowtowing to outrage mobs.