That's such a hard question! So far, some of my favorites have been Deirdre's autumn home, Ribbot's robo-themed house, Eunice's cozy winter cottage, Harry's sloppy home, Canberra's multicultural restaurant, and Digby's kitchen on a cliff by the sea. Agh, I KNOW I'm forgetting some.
My favorite house I've designed so far has been for my boi Roscoe. I actually never seen him before I made his house and I loved it so much that he's now one of my favorite villagers and he's currently a permanent resident of Ashville (my New Leaf town).
There are a lot of cool houses I had fun making, but I'll always have a soft spot for Moose's house- I really enjoyed designing for my favorite secret agent buddy ^u^