Shantae: Risky's Revenge is a decent platformer.
The sequel is available for the Wii U and is pretty good as well. I assume you mean the regular Wii though.
Cave Story is decent if you've not played it. It's retro style but pretty fun.
There isn't a ton of really great Wiiware games. A lot that aren't bad, but none really stand out that I can think of.