Vaettur
Based on what i heard of Fish, Mario vs. Donkey Kong isn't that great. I dunno what you should do though, i never buy WiiWare games and i don;t have a DSi.
I like the Rabbids (?) game where the rabbit is stuck in the Wiimote and when you shake it around, the rabbid gets thrown up against the walls and stuff, and it interacts with the buttons when you press them, and it's SO much fun when you play it with friends, and it's the funniest thing in the world.