This includes graphics of course, but also other innovations in both technology and design.

What say you, and what's your reasoning?

I voted for Gen 5 because of the huge improvements in both gameplay and visuals.  Compare 3D in early 3DO games with how solid Banjo Kazooie or Perfect Dark looked and played.

But we can't sell other Gens short either, like Gen 7 (early X360 games compared to Uncharted 2/3 or The Last of Us) or Gen 4 (Super Mario World vs Donkey Kong Country).

Gen 3 also saw massive visual growth for those who weren't around back then.  We started with games that looked like this...

and ended with graphics like this:

I'd say Gen 3, both visually and especially gameplay wise, followed by Gen 5.

Play some early ps3 games and then play tlou on it. Nothing comes close!

The 360 started with Perfect Dark Zero and ended with Phantom Pain.

The third or fourth. In the third, we started out with SMB 1 and NES baseball, and ended with Ninja Gaiden 3 and SMB 3. With the fourth, we went from Altered Beast to DKC. That's about as big a leap within one generation as you can possibly get.

Looking at PS3 launch titles and then looking at The Last of Us or GTA V is pretty crazy.

What generation is SNES?

Started with pixel sprites and ended with 3D polygons.

I'm also gonna go with Gen 3. Just comparing mid 80s NES games to NES games from the 90s and it almost felt like at least half a generation leap in some cases.

4th gen easily. When you look at the games released at the beginning of that gen for systems like the SNES, Genesis/MD, TG16/PC Engine... and compare them with ones released near the end, its night and day difference. And it was the same hardware throughout. What changed were the creative ways developers figured out how to really make things happen nobody thought possible.

Sure the later gens were better, but the 4th was truly revolutionary in gaming. Everything else after was evolutionary.