For me, now, the absolute best is Mua Pel'am, Sulani. Hands down... <3
Before Island Living, Windenburg Island was my favorite area. I also absolutely LOVE the rural countryside neighborhoods in Windenburg and if Granite Falls was a residential area, my Sims would live anywhere there in a heartbeat.
There's areas in all the worlds I own that I use and love, but those are mostly just a lot here and there, not entire neighborhoods. However, I have a feeling once I'm able to get my hands on Glimmerbrook, that neighborhood will be another favorite. I love nature, the wilderness, remote areas like islands and rural areas above all else. It's just my country girl mentality shining through having lived there my entire life. :)