My favorite series would probably have to be the Gorgeous, Graice, Classic and Rococo series in that order. Yeah, other than the Graice series I really like a lot of old or rich looking furniture. Only thing I don't like is the Classic wallpaper and flooring. I think the Mansion wall and the brown or red colored rugs/carpets.
Have you completed the HHA theme challenge? If not, look that over before you start! I started with the exotic series, and it's still one that I have kept. Ice is great too, altogether or it mixes well. I did not like the Alpine Set at all until I started seeing some of the great customizations. It's replacing everything as my favorite now!
I totally re-arranged my alt's houses last week, and for the first time ever put a princess set out on display..and omg it's soooo nice! I'm not very princessy, but I even I really love it! I combined it with white refurbished weeding day items, and a white grand piano, and it looks awesome! I'm definitely a fan of the princess set now ^_^
kiddie set customized pastel, regal set customized pink/blue, cabana set customized with gold ore, ranch set customized white, patchwork set customized warm colors (looks really great in the fall months), cabin set :3
My favorite common set is tied for alpine and astro. Two sets that have different colors available and look pretty great (in my humble opinion). My only wish is that the astro set had MORE color options, especially green and yellow(yellow and black, how cool would that have been?). Alpine is a little more versatile and I enjoy being able to choose the wood color and make custom designs for the fabrics. Astro is a little less versatile, but I am attached to it anyway because of its futuristic look.
My favorite rare set would have to be the gorgeous set. I just can't resist the red/gold/black!
It's so cozy-looking 
But the Kitchen is a really cool-looking theme too 
