My favorite Wii game hands down is Mario Galaxy 2. It's absolutely the pinnacle of 3D platforming. As good as Galaxy 1 was, Galaxy 2 cuts out the 5-10% of fat that you never even realized was there. The UI has been simplified and the hub world mostly done away with, which seem like losses at first but really just mean that you're spending less time navigating towards fun and more actually having fun. The slightly smaller, streamlined worlds work in the same way -- there's very little dead space. Practically any time you're moving the analog stick in Galaxy 2, you're experiencing sheer platforming bliss.
Similarly, Donkey Kong Country Returns is to 2D platformers what Galaxy 2 is to 3D platformers. It has yet to get any better. It's NES-hard and thanks to some clever effects and truly innovative design the levels feel more alive and dynamic than any other 2D platformer. NSMB Wii is what you want to have fun (and anger-fun) with a bunch of friends, but feels a bit empty and sterile in solo play. DKCR is what you want when you want challenging, intense solo platforming.
For my money, those are the two best games on Wii. But honorable mentions to:
-ExciteBots: Highly underrated and sold tragically poorly, this is a creative, truly bizarre racer that emphasizes points, style and crazy tricks over merely trying to come in first. Motion-control steering is airtight and feels great, and the online play is surprisingly robust. Mario Kart Wii practically felt like it came out of a can; this is the racer made by your weird little brother who's either going to grow up into a serial killer or an eccentric millionaire genius (or both). Or if it's too weird for you, try its less-weird predecessor, ExciteTruck.
-Punch-Out: Also slightly underrated, a fantastic follow-up to the NES/SNES games. The characters trade in the same kind of over-the-top stereotypes as the original, but it's all done with such character and style that it's charming and memorable instead of just offensive. Also, don't listen to the haters, the motion controls for punching work just fine. This ain't an easy game but I got halfway through the second, much harder portion of the game with motion controls alone.
Two more that aren't for everyone:
-No More Heroes: Charmingly rough-around-the-edges and thoroughly Japanese hack-and-slash game that, unlike many modern "cinematic" games, has no aspirations to be a movie and gleefully revels in being a videogame. It was one of the first games to realize that motion control may be better used sparingly, as punctuation rather than writing entire sentences -- basic sword control is done with buttons, but when it comes time to deal the finishing blow, time slows and you're invited to do a big, showy slash with your Wiimote, rewarding you with a fountain of blood and coins and weird, intentionally repetitive death cries like "my spleen!" Sure, you coulda done those finishing blows with a push of an analog stick (and I think that's what the PS3 version did) but this is missing the point; I think gamers in general underestimate how a big visceral swing of the arm can result in better immersion and engagement.
-Silent Hill: Shattered Memories: Basically what Heavy Rain should have been, this is an interactive story that isn't just a string of cutscenes and QTE events but instead tells a narrative that is made all the richer by being told as a game. It's also thoroughly creepy and makes you feel helpless, mostly because you can't fight your attackers at all -- when one shows up, your only choice is to run. Definitely not for novice gamers as about 50% of people are probably going to find the chase sequences incredibly frustrating. Still, I find the overall experience worth the journey. Sure, you can probably guess where the story is going, but after it's over you think about the ending a bit and realizes it recolors almost everything about the game.
As for Wii U games, well, there aren't a whole lot so far. But as I'm only a few worlds into Mario 3D World, I'm leaving my best-of-Wii-U vote to ZombiU. Again, not for everyone, as it's punishing and very unforgiving, and instills you with a sense of hopelessness and loneliness -- both in the game and real life! But it's goddamn thrilling once you get into the groove, and makes a better argument for the Gamepad's existence than any other game out there.