Painter, author, and actress are my favorites, are just about all I do with the sims I spend most of my time with. I love the author and painter careers because I can play those sims as self employed. I don't have them join the actual career path set by Maxis. I do enjoy the active part of the acting career, but just as important, they can set their own schedule. They keep money rolling in, especially the painters, and they have lots of free time to do fun things. Musicians and singers would be great if we only had the EP to support those careers properly.
Paranormal investigator, interior designer, and conservationist are also fun ones but I've only done them once or twice each. I keep meaning to play those more often, but I always stick to my routine.