Lots of SNES games still look brilliant. I think FFVI looks better than a lot of the 3D FFs, and Yoshi's Island still looks positively amazing. I still like the look of X-Com: UFO Defense and Jazz Jackrabbit, too.
The problem with old games is always image quality. I don't think you really get the beautiful 2D games until 8-bit color came along (maybe a few exceptions), and with 3D, pixellation and PS1 wobbles looked bad even then. I mean, I remember the first time playing Wolfenstein and wondering why everything looked so terrible up close. PS1 just made me wish 2D consoles would stay around a little longer.
However, once you hit the era of bilinear filtering, I think you get some games that still look really good. Rayman 2 on the Dreamcast still looks incredible, and I think Mario 64 is still very pleasing to the eye. Quake II still looks pretty good IMO, although it gets repetitive. I actually like the look of Turok 2 on the N64 a lot (PC version lacks some lighting), but the frame rate is kinda crap unless you have an emulator.
Is Serious Sam old yet? I think it looks great, too.