Definitely Gorgeous. Gracie set is too weird, Sweets set is too overused and boring IMO. I like the Princess set too, but I like the Gorgeous set more. It just gives me that nice cozy feeling since it's the winter series.
To add on to what I said earlier, I like the Gorgeous set because it can have so many different feelings to it. It can have a fiery feeling like in Phoebe's house, a regal or fancy feeling like in Zell's house, or a warm and cozy feeling like in Lionel's house.
My favorite of the Gracie furniture series would be the Gorgeous Series.
The Princess Series is my second favorite. The Card Series and Gracie Series are both fine, and I like them, but not as much. I don't have any strong feelings for or against the Sweets Series.
I like the Princess series best, because blue is my favorite color and it's very classy. I don't like that it doesn't match well with any other furniture set, really. (The Regal series customized to pink or blue looks OK with it) That often means seeing yet another room that's full of nothing but Princess furniture.
I really love the Gorgeous chair and sofa, and the Card tower. The Sweets series is adorable, but I don't use it because I like the furniture in my homes to look like actual furniture. I've seen people do great things with all of the series, though!
I like the Princess set too and that the sweets has a music player and is also pretty but my favorite is the Gorgeous it just looks so really upscale like Gracie type store might really be it reminds me of a Gone With the Wind look with that fabric and color. So gorgeous my least favorite is card then gracie.