Little King's Story on the Wii.
I'm not kidding: Don't even look at the packaging. Don't let ANY preconceptions about the game alter your ideas of what it might be. It is absolutely a GOD-tier game, and I can't possibly sum up why that is so without a full-on essay. If you can get past the cuteness at all and you own a Wii, BUY IT NOW. It's not only better than most 3rd party Wii games, but better than most FIRST-party games. Dying for some Pikmin 3? Give LKS a try. (that's the closest comparison you can make, really, but it's still quite different) There is nothing else like it out there, and the game got NO attention at all. I'm not joking when I say it should have been GOTY when it came out.
(also, don't judge the game on what you see in the first few hours, either. It builds up slowly but where most games would stop piling on new things, LKS just keeps going deeper and deeper, more and more intriguing and difficult, crazier and more thought-provoking. You have no idea what you're in for with LKS. You cannot say the Wii has no good games if you have not beaten this one.)
Edit: I realized this is a topic more about games people played but said were bad, but I guess with all the bad rep the Wii gets, I could put the whole damn thing on here as my pick for most underrated. Especially 3rd party on the Wii. The adoption rate for the Wii's shining gems of 3rd party is insanely low compared to the worse first-party games. Like Mario Party 8 or Wii Sports Resort. So many Wii owners out there own like 5 games, all first party. They're ignoring not only a nice handful of the first-party stuff but a LOT more. Sure there's a lot of casual crap on the shelves, but the good 3rd party stuff can be really GOOD. Also a good number of truly HARDCORE games, which I don't usually see many of on the HD systems.