Back when I first started playing, I saw a streetpass house with the Princess furniture set and I LOVED it. It’s probably been the only real favorite of mine, but now I love so many different pieces put together in one room I could never just have one set. I love the Alpine set a lot too. It is fun to see what goes together but I don’t always score the most points with the Happy Homes Academy lol.
If I had to name one, my favorite is probably the kiddie set. It's hard to pick just one because all the items in AC are super adorable. I also enjoy the lovely set and the Gracie set.
For me, it would have to be the Kerokerokeroppi set. It is so tranquil and different. I love it. However, in a full sized room it doesn't take up much space and doesn't include items such as a bed (although it takes more room up if you display it with the outfit) so I like to mix it with other sets like the cabana set which ties in so nicely with it.
For orderable sets, it's either the astro set (refurbished blue and black) or the minimalist set.
For non-orderable sets, I'd probably go with the ice set.