Wii U - not underrated, it has a small but high quality lineup and is a gimmick console. If you're not into the nintendo-franchises, it just isn't for you.
Saturn - not underrated. Lacked 3d capabilities at a time where it just started to boom. Sega should've seen that. Western software lineup was overall pretty meh. If more Japanese titles would've been ported, it eventually would've sold better.
Xbox - ha, no. Sold rather well for it's late-to-the-party arrival and its short lifespan.
Gamecube - yeah, kind of. Not supporting standard DVDs was kind of stupid. Missed a few killer titles at the beginning and was overshadowed by the PS2.
PSVita - definitely underrated. Impressive technology, great small lineup, but lacked the exclusive killer titles. Should've come out 2 year earlier - and Sony didn't even try to save it.
I'd say the most underrated is the Dreamcast: Strong lineup, great technology, overshadowed by Playstation brand. Customers lost faith in SEGA due to their previous missteps.