Let's see here...
I'm a big advocate for Swords & Soldiers - I know it's not everybody's cup of tea, but as someone who generally struggles with RTS, I found this to actually be very approachable and accessible. Also, there's a lot in there, so you're certainly getting good value for your money.
On the platforming front, your options for good'uns are plentiful. There's the aforementioned Mega Man revivals, though I agree that MM10 is a significant improvement over MM9. Outside of retro stuff, there are the two LostWinds games - if you're going to play just one, go for the sequel, Winter of the Melodias, which is superbly designed and visually striking. In that vein, you could also check out NyxQuest: Kindred Spirits, another puzzle platformer that shares many of the same qualities as the LostWinds titles. You might also try a really unique one called And Yet It Moves, as well as Toki Tori, one of the best launch games for the service that may have been sadly overlooked. Speaking of unique and sadly overlooked, definitely pick up Fluidity/Hydroventure. Oh, and don't forget Cave Story. Finally, moving away from the 2D side, I'd also recommend Jett Rocket.
Naturally, we can't properly discuss WiiWare without giving due dividends to the Bit.Trip series. I personally found Beat, Void, Runner and Flux to all be highly enjoyable, with RUNNER being the standout amongst them. Don't know about the other two. Maybe the upcoming 3DS compilation is the way to go.
What else? Most would give World of Goo the coveted accolade of 'Best WiiWare Game', and I have to say, the argument for it is compelling, what with the interesting physics-based gameplay and excellent production values. If you're into Bomberman, you're unlikely to do better than Bomberman Blast. There's also a slew of great puzzle games, including Art Style: Orbient, Art of Balance (incredibly addictive)and the underrated You, Me and the Cubes. Last but not least, I recently bought FAST: Racing League; I've had hardly any time to play it, but my initial forays have been positive.
It's a shame that the rubbish has coloured people's perception of WiiWare, because when you start to dig in deep, there are some true stand-outs.