As the header says, what retro game (and by Retro I mean before the Sega Dreamcast) do you believe looked so graphically awesome you had a hard time believing that it was running on the hardware you were playing it on?
All set? Good!
In my mind, that would be Battlezone for the Atari 2600. What? You're kiddin'! How could you get a classic vector-based FPS onto an Atari 2600? For those not asking that question right now (and haven't seen the arcade Battlezone in action), here is a video showing the original Atari classic;
(Arcade) Battlezone (via SMacReBorn)
And no, you can't drive into the volcano. But if you've seen the average Atari 2600 game, there's no way you're going to put this game onto the system. So what do you do?
You get creative.
First, no vector graphics. Second, create gradient colors for depth effects. But keep the FPS aspects.
And here is what you'll get:
Atari 2600 - Battlezone (via Willowbob)
Granted, you still have the standard Atari 2600 sound effects, but the game itself looked and played amazingly well. Even today it plays great (if you don't have an Atari 2600 and the cart, the Atari 80 Greatest collection on PS2 has it, as well as the original arcade version; I have the PC version on my MAME cab). It just looks so good on the 2600, I was amazed when I first saw it. And I used to play the original in the arcades back in the day.
Okay, I've said my piece.
What games would you suggest looked SOOOO incredible on it's system you couldn't believe it wasn't on a more powerful system?