Fallout 1, I didn't play too many rpgs before Fallout, most of them had a fantasy setting and I didn't like fantasy too much back then. Fallout blew my mind. This game really felt alive, I haven't played a game before that allowed the player that much freedom and offerd so many diffrent ways to play it. I was really impressed when I talked with my friends and compared how different our experiences with this game were. I also liked the athmosphere of a dying world, beautiful music and artstyle.
Another World, I wasn't into games that much back then, and most of the stuff I played can be described as "kiddie games". Another World really seemed like serious business in comparison. The realism, a character that looked and moved like a real human being instead of some shitty cartoon character and he could die really easily. I also liked the variety this game offered, in a normal platformer when you played through the first level you knew what the rest of the game is going to be like. Not here, Another World kept me guessing whats going to happen next. The cinematic cutscenes were also a thing I haven't seen before, I watched that intro every time I started the game. Of course there's also the amazing athmosphere and aesthetics of this game. I remember being really creeped out, kind of scared by it, some weird combination of outlandish-ness, violence and totalitarian undertones.
Doom, the usual Doom stuff: novelty of the fps genre (I played Wolfenstein before but I wasn't too impressed with it), horror like feel, violence, visuals.
Frontier, I enjoyed playing Privateer more but Frontier had this huge and realistic universe and the ability to seamlessly enter the athmosphere that was really impressive. I remember that before playing it I was thinking that it would be cool if there was game that had this option but I also thought that this would be to impossible due to the technical limitations of computers. And that's the reason there aren't any games like that.
Dune (the first Dune, not the RTS sequel), mostly because of the novelty, there was nothing like it. Also incredibly athmospheric with an interesting story (though I already knew it from the David Lynch movie)
Wipeout 2097, it had a sort of "hypnotic" effect on me. The high level of concentration required to play it combined with extreme speed and awesome electronic music, I felt like I was in a "trance-like" state while playing.
Wing Commander 2, first space combat game I played, made me go "whoah" mostly because I have never played anything like this before. I also really like spaceships and military sf so this was like a wet dream to me.
Independence War, just the awesome and novel gameplay. I was always wondering what a more realistic space combat game would be like and here was my answer.
Just from the top of my head, I could probably list a list a lot of games here. Generally I'm impressed by games that introduce some new type of gameplay or provide a interesting twist or refinement of the old stuff. Doesn't hurt if they're also athmospheric and stylish.