The sixth for sure... I find Star Fox Assault looks as graphically impressive as ever...

Probably 4th gen, particularly the SNES although the PC Engine, particularly the Super Grafx also represented the pinnacle of low res bitmapped graphics.

NES. After a few hours, my eyes can adjust to any gen.

Gen 3.

SNES and Gen 6.

Its funny because SNES looks fine but PS1/n64 look horrible. ps2/xbox/ngc erra look ok on a non-hd-television and ps3/x360/wii u upwards look fine overall

Still rarely play 5th gen games. I've only played earlier than that a handful of times.

It is interesting that when the PS1 and Saturn debuted, 3D polygon graphics and environments looked so revolutionary even though the renderings were incredibly basic. Visually, it was arguably a step back from the super clean bitmap graphics of the 16 bit generation even as 3D graphics allowed for advances in game design and play. 

The more I think about it I honestly think I prefer the graphics of gen 3 and 4 more than 5 and 6 for the most part. Games that were attempting to look realistic prior to gen 7 really have aged rather terribly for the most part.

There's plenty that look good, but really the only group of games that I occasionally have trouble getting past the visuals with are those early polygons and horrible textures of many gen 5 and 6 games.

NES and SNES gens still look great. I would say it's because, by their nature, pixels tend to age better than polygons, just the aesthetic appeal is greater. Looking back at the N64 now, while I wouldn't say those games were ever BEAUTIFUL to look at (nor PS1, nor Saturn, at least the 3D polygonal ones, what once seemed impressive is not rather ugly. PS2/GC gen are better looking, and hold up better, but I'd still say that overall polygon graphics hold up far less well than sprites, especially given greater expanses of time. 30 years old sprite games on the SNES still look gorgeous. Mario 64? Not so much.