Like I said, Sega hit its own personal high point with the Genesis, before falling (Sega CD) .... and falling (32

.... and hitting rock bottom (Saturn). Dreamcast was a good system, but too little too late. Getting back to the Genesis, it and the Super Nintendo brought in what I'd consider the 2nd golden age of video gaming. But Genesis could not match power with the SNES, nor could it match 3rd party support. The only area where Sega truly killed Nintendo were sports games. The Genesis is up there, but there are more great games for Super.
The NES is the greatest system of all time as far as longevity and nostalgia. I'd like to give that nostalgia distinction to Atari, but many more people today will remember a lot more about the NES. Although many games pale even by SNES standards, as I detail on my site, they had a certain specialness and flat out screwiness that can never be duplicated.
Regarding Atari, that's where I started. I was learning to play Super Breakout as I was learning to walk. But in giving life to the video game industry, it would have single-handedly killed it if not for Nintendo. But still, nothing beats Atari as far as simplistic yet quality fun is concerned.
Intellivision? You liked those phone dial gamepads?
Still, however, I give it to the SNES. That system encompassed both the franchises and great ideas of the systems before it, and also paved the way for everything we have today.