I think I would have to go with NES, actually.
Seeing some of the other answers so far (Saturn, Atari), I wouldn't be able to call them overrated, because most of the people I talk to don't have high opinion (or any opinion at all really) on them. You'd have to look at thinks that generally have a positive reputation, which really only leaves you with the NES, Genesis, Dreamcast, N64 and SNES, and I think most people (and not everyone) think that Genesis was a good, but possibly not great console. I personally think the Dreamcast is a little underrated, which leaves me with NES, SNES, and N64.
I'd say the N64 is out right away. Its got some fantastic games, but a ton of unmitigated crap, and not nearly as strong a library as the other two.
They both had their share of bad games, but I think the SNES edges out a bit because it didn't really have as much hardware problems as the NES. At least for me.