PS1 era it would have to be Ridge Racer Type 4. The only racing game that looks better than Gran Turismo, and that's quite a feat.
Skipped PS2 and went straight to PS3, and my wow moment was definitely GT5P. Honestly I would be happy if that's the limit of graphics for racing games. If you show pictures of GT5P to non gamers even today honestly I'd say 95% wouldn't be able to tell what is real or not. Special mention to Blur for its unique art style and presentation. It's one of those games where even just looking at part of a screenshot you'll know instantly where it comes from, just like Automodellista on PS2.
Current gen I gotta give it to Polyphony again. There is just something about the lighting in GT that is just so subtle and realistic. Other games might look better in photomode or zoomed in but in motion GTS is so natural. Special mention to Driveclub's weather effects and day/night transitions and NFS 2015 car models in garage.