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...
Mighty Switch Force
Dillion's Rolling Western
And I'm pretty sure there are some gems in DSi Ware land that I don't know about yet.