I'll repost my wall of text from Kotaku comments section here. I highlight a lot of great games, many of which really went under the radar.
First off, I start with my favorite Wii game and one of the biggest under-the-radar gems I even know of. Little King's Story. It looks childish but it's a really quirky presentation that at first might have you wondering what the hell you just bought, but by the end you'll wonder how such a game could ever have worked without it. Gameplay is quite reminiscent of Pikmin, but it's definitely far more unique than that might sound. You go through, building up your kingdom and traveling the land to uncover new people and places and the game as a whole just gets deeper and deeper. It's also a lot tougher than it appears. The boss fights are ALL some of the best and most unique I've experienced in any game. The story is great and both heartwarming and sad at the same time, making for a package I just couldn't NOT recommend. There's nothing else out there that could compare to this game, and probably never will be.
Mario Galaxy 1 and 2 are solid choices, in the first-party category. The amount of creativity and variety in the gameplay is absolutely top-notch, and it doesn't look half bad, either.
New Super Mario Bros Wii, either for the sake of some true old-school tough Mario platforming action singleplayer, or one of the most fun games that any friend of yours can easily hop into and have fun with multiplayer.
Boom Blox also makes for a great party game, if you have some friends to play with. It's like Jenga, but with a lot more tower falling stuff.
You throw baseballs/bowling balls/explosives/etc at towers of blocks to knock them down for points in one mode and you carefully pull blocks out of precarious towers for points in another. It makes great use of motion controls and has plenty of play modes and build-your-own stuff. The sequel also lets you download levels online from other players.
Speaking of great uses for motion controls, Elebits is one to look out for. It's all about trying to hunt down these little critters hiding in your house, yard, neighborhood, etc by using your gravity gun to move objects around and zap them away to power up the town. Your gun starts out weak and there's no electricity at the start of each level, but as you build up power by capturing the little things, objects start powering up. By activating electronics, yellow/pink ones fly out, which power up the weight capacity of your gravity gun. So it's a combination of first-person shooter and flinging stuff everywhere to make a gigantic mess. Levels mix things up with elements like zero gravity, noise limits and a limit to the number of objects you can break. The controls work really well, though, allowing you to push, pull, lift, twist and move objects simply by making the same gesture with the remote.
Zack and Wiki also has some nice uses for motion control, though in this case I'd say it's a bit more hit and miss. It's a point-and-click puzzle game, where they mix things up quite a lot. Sometimes the challenge is in figuring out what order in which you do things, and other times it's about finding that perfect solution to get around something. Sometimes it's about trying to work out how a strange and somewhat complicated machine works and other times your solution is a forehead-slapping obvious motion you just never thought to try out. The game doesn't hold your hand or make things easy for you ( unless you go out of your way to ask, which results in a game whose puzzles can perplex you for hours but make you feel awesome every time you figure it out.
If you're a fan of light-gun shooters, you're in for a hell of a time. Resident Evil: Darkside and Umbrella Chronicles are both excellent games that contain a lot of Resident Evil lore and are generally well-made and a lot of fun. Dead Space: Extraction makes one hell of an argument for Dead Space being on-rails. I'd almost say it's right up there with Dead Space 1. And there's some solid House of the Dead action to be had, which... need I say more? With Wii pointer controls, all these games work tremendously well.
Similar, but different: Sin and Punishment: Star Successor. It's kind of a mix between light gun game and Star Fox, really. It's awesome and painfully, brutally hard. Luckily there are checkpoints between most bosses (good, because you'll be fighting them 3 in a row in most levels- this game has a TON of awesome boss battles) because you WILL die, and die a LOT.
Speaking of bosses, Monster Hunter Tri. There's not much story to be had here, and the gameplay is truly deep and complex. You won't be able to hop into MH without prior experience and expect to do well. The tutorial section lasts a good two hours as you learn the ins and outs of not just beating the crap out of mega-sized crazy monsters (trust me, after hour 2, this is 90% of the game) but how to gather resources, craft invaluable items and tools, perform upkeep on your weapons, hunt smaller game and carve up your kills. There's a lot to do which really adds to the "hunter" part of the game. You really feel like this is a living, breathing world, and it comes alive in every detail from the gorgeous visuals and realistic animations to the very actions you perform. And of course, the bosses are an absolute triumph of action gameplay. They will murder you in a few hits at first, and even the first big raptor boss will have you shying away from ti at first as it mauls you the second you get close. But you learn the attacks and tells, figure out what items, weapons and armor (there's a really deep armor and weapon skill system here, without character levels at all) to bring with you, and it just feels great when the beast finally goes down and your hunt is successful. Multiply the awesomeness by 4 with the robust online system and you've got a 200+ hour game right here. If you've never played Monster Hunter before, there's no better place to start than Tri.
Cave Story on wiiware. You probably have a PC, so you can probably download and play that version of this game for free right now. But I really like the Wiiware one, and it's a fantastic game no matter how you play it. Pixel deserves the $10 for this, though, and I think you'll agree. It's the kind of game that would have been amazing had it existed back in the SNES days, but holds up perfectly today. (well, it's not too old actually...) With the updated visuals, new music, extra game modes and difficulty levels and ability to switch back to retro completely, it's completely worth the price.
Donkey Kong Country Returns: if you like platformers, the DKC games on SNES, or just another really hardcore game, this is for you. Really, between this, Monster Hunter, LKS and S&P, there's a lot of true hardcore games on the Wii, most of which could rival Dark Souls in difficulty level.
No More Heroes 1+2. Great action games, awesome humor, and a good style that uses the Wii's power in the right sort of way. They both tend to drag on a bit, but the boss fights are what it's all about, and those are well-done and satisfying.
There should be a number of pretty obvious ones out there as well, like Mario Kart Wii for instance. You probably already know if those are right for you. There's also a lot I've yet to get to or just haven't finished yet, like Fragile, Red Steel 2, Deadly Creatures, Muramasa and plenty of downloadables. A lot of these games suffer from their own flaws but most of those can be overlooked because the experience is still a really unique and fun one overall.
