Definitely NOT Gamecube. It is almost universally praised among all my gaming friends and no one has much bad to say about it. Maybe in its time it was hated (mostly ignored I think) but it gets so much praise especially for its controller that anyone who calls it underrated apparently thinks it's the second coming.
I would say Wii because it's drawn the most ire from gamers for being the "Wii sports" machine despite some excellent (though I think sometimes overrated) software. It matches the 360 catalogue imo and doesn't draw nearly as much praise.