It's so hard to choose! Lol
The Mermaid set is my favorite set introduced in new leaf, but the Regal set has always been my all time favorite (I wonder if anyone can guess why, lol!)
The customization options introduced in new leaf made the Regal set even better! Royal pink and royal blue make them extra special.
There are far too many series I like: Regal, Alpine, Sweets, Rococo, and Gorgeous, to name a few. But if you put a to my head, I'd choose the Princess set. I wish the colors could be customized, but the fancy white and blue is still beautiful.
The mermaid set is pretty sweet. I remember working really hard to get it, though now I forget how you did. One of my rooms in my house in ACNL is dedicated just to the mermaid set, and it's the room I picked for one of the events. Mermaid 4 life.
it's so hard to just pick one omg there are so many great sets but off the top of my head some of my favourites are the sloppy set, mermaid set, insect set and sweets set!
For entire sets I think my favourites would be the Regal set and the Princess set! I also really like the Ranch series, I think its really nice for kitchen and living rooms :>
The Rococo set and the Princess set are my most favorite furniture sets in New leaf. I really love their aesthetic, especially the blue variants of the Rococo set.