I've updated my opinion lately because on new towns, so I'll just explain my opinions.
Riverview: Riverview is still my favourite town without a doubt. I love the size of it, all the empty lots, the farm/country feel and the range of townies. I live in England in a large town but I have always wanted to live out in the countryside. I can bored of endless streets of modern houses, roundabouts, cars and people. I get tired of walking down the same pathed redway to school every morning and walking down the same path back every afternoon. Riverview is kind of my dream place to live. It reminds me of villages I drive through and where my grandparents live. If I were a sim, I would live in Riverview. I also like Riverview for the empty lots to build on. Most of my contest and swap shop builds are completed in Riverview unless stated to be done in other worlds.
Appaloosa Plains: I love Appaloosa Plains for similar reasons to Riverview. The empty lots are big, but a builders dream. I don't see myself living there, mostly because of the desert-type style, but it's a livable place for wealthy sims. There is a huge lack of affordable housing, but a decent builder can quickly solve that. I don't play the townies that often but I love playing with pets and exploring cats, dogs and horses.
Moonlight Falls: The gloomier atmosphere makes a change to Riverview and Appaloosa Plains, but this town is very interesting to play in. The townies are all so diverse with rich story lines and history. There are so many decent houses in this town, with plenty of space to add more every time you play. I love playing witches and fairies and this town is specially crafted to suit their magical needs. To get the full experience, you do need to set the moon phases to rotate, but since one setting is the same for all game files, I can't turn it on since I turned it off for my dynasties. I do kind of regret some of my dynasty settings, but I guess I have to live with them now.
Union Cove: This world is very diverse with lovely townies, beautiful houses and landscaping and very creative lots. There is a lack of empty lots, but that can't really be helped and you just have to sacrifice a lot or two if you want to do some building. I don't play Union Cove often, but I still love the world to pieces.
Isla Paradiso: I don't own any store worlds, so this is the first world where I can explore the tropical lifestyle. I love the Caribbean theme and the rich range of townies. The houseboats are really cool, the dive lots are good and the resorts are stunning. This is basically my dream holiday! I haven't played here much but I do really love so much about the world.
So there we have it. A detailed overview into my favourite worlds.