There's definitely exceptions to all of the gens. Even in gen 2 there's games like Pitfall II, defender, H.E.R.O., River Raid and others that still look plenty good to play even though they're remarkably simplistic visually. I'd say that you don't start seeing a lot of good looking ones until gen 3 though, and probably not consistently until gen 4.

I am not bothered in the slightest by old gen graphics, from whenever. Everything can still look good to me, because I'll have a 2nd, 3rd, 5th gen or whatever mindset when playing such a game. I set my expectations as such.

Now if you're asking me what gens still look as esthetically appealing now as they did back then even compared on equal grounds, picture to picture, to the current gen, I'd say the 4th gen. Some games from back then aged so well, the best ones like Yoshi's Island even look prettier to me than most modern games!

Its hard to say cuz games from the NES/genisis era were mostly cartoony so they still look alright now as it doesn't really age, it was when games started going into 3D and those early 3D games that look really bad, they're pretty much unplayable and I wonder how I even looked at the screen for that long it's that bad.

Thrillville on my PS2 still looks awesome!

For me, the "16-bit" era. Games like Mega Man X and Super Mario World still look amazing today, and if it weren't for their dated sound, could easily pass for brand new indie games.

Gamecube, all the way. Windwaker, SM Strikers, TP, Sunshine

That is true. I completely agree. 

The 3rd gen already have some very nice looking games.

            

Depends. 2D games have looked great since the NES

As for 3d...eh, maybe gamecube? N64 was too simplistic for me to consider it fine.

Though there are some games that still holds very well on the third gen, overall speaking the 4th gen still holds superb and probably will be for the end of time.