The 6th. I'm a relatively young person, so I grew up on the 5th and 6th generations, and while, probably due to me being a completely care-free little child back then, I have more fonder memories of the 5th generation, but as I am here today, I can say that most of my favorite games come from the 6th generation, and the PS2, mostly.
Also, the 5th generation saw the beginning of the Playstation, so when the 6th generation launched, with the next Playstation, Microsoft making a console etc, then it was pretty exciting.