I honestly think it's the sixth. I was too young and/or immature to truly appreciate the 4th generation's gems, so I can't really say that. The 7th is excellent, but the huge amounts of bugs and pay-more-for-patches are annoying...plus local multiplayer is dying out.
In the sixth, we had the clear powerhouse of the PS2, but the other systems had their contenders as well...the Xbox had Halo as the fallback and generally better hardware, while the Gamecube was largely inferior but had a couple of grand gems in the forms of SSBM, RE4, Windwaker, and a couple others. Plus it had the Gameboy Advance and the SP...man I loved my SP.