The Sega Dreamcast have been called underrated for so long it has become overrated. The thing is, it may not be underrated now, but in it's time people missed out on a great console. Dreamcast came out far earlier than PS2 or Xbox, and had similar hardware. They also had a lot of great titles, it just never got the love it deserved, because after other consoles came out Sega just gave up on it.
I don't understand why Nintendo 64 is not talked about among the greatest consoles of all time. It moved us into the 3D era seamlessly, as Mario and Link seemed like they had been in 3D all along. It brought plenty of classics from Super Mario 64 to Super Smash Bros.
Gamecube was overshadowed by PS2, the all-time best selling console, and Xbox. It is the worst selling consoles in Nintendo history next to the Virtual boy, and yet it was still a great console. It had some great first-party games, and some games that I would put up as some of the best Nintendo has ever produced. Its cute little purple design was different too.
Handhelds are grossly underrated, because a great handheld can take the world by storm. I would say Gameboy Color was a great handheld. In a world of 3D gaming it managed to carve out a niche for itself and sell like crazy. The Pokemon series pretty much carried it on it's back, but there are a lot of Gameboy Color games that I could play for hours.