Streets of Rage 2 feels remarkably good to play today. Homeworld 1 and 2 are great to play today, too, especially since RTSs are so rare. The remasters do clean up the visuals a lot, but for the time gap between the originals and the remasters the originals hold up amazingly well.
And then puzzle games. The TGM tetris series may as well be a new release. Puzzle Bubble 1-4 actually look and sound better than the new sequel that came out this year.
And if we fast forward to more modern 3D games, Batman Arkham City, which is now twelve years old (damn) would be a perfectly current open world game if released now, and Burnout Paradise, which is now fifteen years old (double damn) is such a nice racing game that when they remastered you could barely tell the difference.
Oh, and Eye of the Beholder. I have no idea of why Eye of the Beholder, of all games, is just as good as it was at launch and as good as modern reimaginings. The UI is so smooth and the game teaches itself perfectly. I could use an automapper, I suppose, but if you ask me whether it or the Etrian Oddyssey games feels more like a contemporary design it's EotB any day.