Omg I don't know if I can pick a favorite. I really really liked designing Tortimer's house because I made it reminiscent of the island on ACNL. I really like the fact that you can change the season and pick different plots. I only recently got HHD (Christmas FTW) but I really really like it so far.
Designed Coco's ancient artifact house recently, and it came out great. I had fun mixing and matching different gyroids to blend with the songs I've used and her interior is a full of artifacts and fossils but I've made a maze to make the exhibit more interesting. And it fits Coco so much.
Alpine, cabin, and zen houses were my jam. But I'm weak at serene and ice houses.
I was surprised to have the most fun designing Bangle's house. It became this tropical fortress & tbh her furniture was so cute. I definitely put the most effort into her's & Ankha's homes.