This is a big topic on Reddit r/gaming right now. I was trying to remember what mine was... I'll go a bit more retro than most are so far here: I think Duke Nukem 3D or Quake was very impressive to me after pretty much only playing Doom. Graphics that used to impress me as a kid were ones that changed the norm up from what I was used to seeing. I remember Mortal Kombats being pretty impressive, since they used real images of people instead of artwork. I also remember thinking Donkey Kong Country's 3D rendered 2D sprites were very impressive back in the day too. More recently, I was really impressed with Street Fighter's HD sprites when compared to the originals, those definitely made me say wow, when you appreciate how far HD quality improves over the old sprites.