Definitely not PS2, because FFX looked great early gen.

Maybe PS1. Early PS1 games looked like trash, later PS1 games could almost pass as PS2 games.

Heck compare 1998 Spyro to 2000 Spyro. Massive difference.

Now compare 1994 to early 2000s.

I don't really have to qualifications to say since I really only started to game during the 6th gen so based on my personal experience, Ps3/360 were the largest. Gears of war compared to Gears of war 3, Uncharted to Last of Us/Uncharted 3, Resistance: FoM to Killzone 3, Forza Motorsport 2 to Forza Motorsport 4, Cod2 to Cod: Ghost, GTA4 to GTA5, Mass Effect to Mass Effect 3, Halo 3 to Halo 4.

C64 - first games looked like something from 1970s consoles, later were much closer to NES.

PS360 come to mind as well.

The diff between Super Mario Bros and Super Mario Bros. 3 immediately came to mind

People keep saying PS360 because that is what they remember but late gen titles had terrible low fov, resolution and fps, some of them nearly unplayable. These systems were oversqueezed and started to buckle under the load.

3rd gen 8-bit system like C64, NES, Master System went from basic single screen action (Tetris, Donkey Kong etc) to open levels, multi-layer parallax scrolling (no dedicated hardware) and moving large, animated sprites. SNES went far too, but it did require new hardware like the SFX chip so they did cheat a bit.

Compare Metroid NES, one of the most complex NES titles (early-ish) with its simple, tiny sprites to Bucky O'Hare with its large moving sprites.
18:30
https://youtu.be/idiM8gGIAAI?t=1113

...or Batman 3 with again multi-layer parallax scrolling and large animated sprites
https://youtu.be/p1Ivxnpbd5Y?t=1063

C64 Wrath of the Demons with parallax scrolling and large animated sprites
1:00
https://youtu.be/j2u3vNfW3a4?t=59

The NES went from Donkry Kong to Super Mario Brod 3, which looked better than some 16 bit games.

PS1 went from blocky-ass fighting and racing games to games like FF8-9, Gran Turismo, and Ehrgeiz.

For me, it's gotta be the Xbox 360. It went from games like COD2 and Perfect Dark Zero...

To the likes of Gears of War 3 and Judgment, Halo 4, Crysis 3, GTA5, Battlefield 3, etc.

There's almost a generational gap in looks between it's 2005 games and it's peak in 2011 and beyond.