I played the GameCube and PS2 for a little while and then went back to my PS1 to play Tony Hawk's Pro Skater 2. That's when I realized "what the heck happened to my Tony Hawk game?" I thought it was scratched or something, but as I played more PS1 games I came to the conclusion that GC and PS2 ruined my past view of 5th gen graphics. I could see the jaggies and block shaped polygons and all of that when THPS2 got popped in.

It was horrible to say the least, but I still enjoyed playing the games anyway (I still do a little No Mercy from time to time).

  What was the first game you saw on the PS1/N64 that showed to you the graphics were bad after you played a 6th gen game?

Lol, Perfect Dark oddly enough, but it saved itself when I got an expansion pak.

In most of my N64 games I never really notice for some reason, I could go back to them even if they were wire frames and not really notice, perhaps because I was so used to them.

Perfect Dark however I bought relatively recently, only 1.5 years ago I think, so going back to it was terrible.
I didn't know I needed an expansion pak to play most of the game though, and once I bought one I couldn't believe the difference it made.

Gran Turismo 2 gave me headaches & vomits after playing Gran Turismo 3 on Ps2 at my cousins house on a holiday..

Tekken 2 for the PS1. After playing Tekken 5 for the PS2, I went back to playing Tekken 2 over my cousins house and noticed how bad the graphics were.

PMttyd after finished it, played the original,
it wasn't totally bad It just looked more blurry and PMttyd looked more clear and faster to me.

Final Fantasy 7...

lol. I used N64, so the presence of choppy graphics wasn't nearly as bad. Poygons? Yeah, those are blatantly obvious now, but it still isn't bad to look at.

It was Mario Kart 64... I downloaded it from the VC, and the graphics are very bad...

But there are some games that have aged extremely well... Star Fox 64 is still amazing.

And... I think Star Fox 64 has the best quotes ever...

I would say FFVII, but I thought it was ugly the day I bought it. Thanks to Square and Sony's BS marketing campaign there wasn't a single screenshot in a magazine showing the actual out of combat character models.

On the N64 it was Goldeneye.

That said, some games that I can still go back and play today and enjoy just as much: FFIX, OoT, MM, SF64, MGS.

Silent Hill, really showed it to me. Most of the N64 games didn't do as much shellshock as any of the PS1 games I've seen since Gen 6