Corncob3D - First flight sim I played, and probably my first 3D game. No textures, just plain colours and running on DOS in 1992. Absolute blast, though. I think it was pretty much the reason I got a joystick back then. Also, it had permanent death, which is not something that has been explored overly well in flight/space sims. Getting downed behind enemy lines meant humping it back to your airstrip with nought but a pistol and a map, and there was really a sense of urgency to save your veteren pilot for another purple heart.
Fury3 - My first space sim. The first computer game I actually put on my christmas list too, I think. Played it with an ancient joystick on a machine that could barely handle it, but it sure was fun. Debris and particles were great, and following your spiraling, stricken enemies down through cloud cover until they slammed into the terrain below was an extremely rewarding experience.
Wing Commander - I didn't really play this a great deal - a friend of mine had a copy, so we used to just fly around a bit whenever I went over to his. I picked it up again years later in DOSBOX and it definitely was still a blast.
Legend of Zelda: Ocarina of Time - A game with a storyline! Unheard of! I'm pretty ashamed to say I needed a walkthrough for the Water Temple. I still stand by this being the best RPG ever created.
Diablo2 - I didn't play this until years after it came out, and it really blew me away. The art and atmosphere was fantastic, there was very little grinding in the single-player campaign, and the story was engaging. It was really the dark, melancholic feel of the game that hooked me, though. When I got to the end of the first act and the sight of the tortured-to-death-and-forgotten corpses in the Monastry didn't even phase me, I knew I'd been playing too long.
Freespace2 - Now, the order of things here was weird. I bought a new joystick - a Wingman Force 3D - though I have absolutely no idea why. I knew of no decent space sims at the time, and wasn't interested in flight sims. It came with a copy of Freespace2:The Colossus though, which I almost didn't install. I think I ended up playing through that demo 20 times before I managed to track down a copy of the full game. It's worth noting that a mate of mine bought me a boxed FS1 with manual after he stumbled across it in a second hand bargain bin for AU$6. People just don't know the value of what they sell.
Nothing's really "blown me away" in recent times, though. There are plenty of games I've had fun with, certainly, but nothing that I'd expect to be looking at through rose tinted goggles in ten year's time.