They're all great, though my top three would be Pushmo, Zen Pinball and Mighty Switch Force - in that order. On DSiWare, most ArtStyle games, all titles by Q-Games, all games by WayForward and Rytmik are definitely worth a look.I played Mutant Mudds demo yesterday and it'll probably be the first eShop title I buy. Though now because of that title I want to get some titles that have caught my eye...
Pushmo
Mighty Switch Force
VVVVVVVVVV
Zen Pinball
Sakura Samurai
Dillion's Rolling Western
And I'm pretty sure there are some gems in DSi Ware land that I don't know about yet.