Which careers and majors do you have your Sims pick the most? I personally can't answer for myself; I haven't really actually PLAYED my Sims, mainly only for stories and spamming my family bin. But I'm about to! :P
From the Maxis careers I often use the Education, Science, Law & Law Enforcement careers, for some reason. Custom careers that I often use include my Falconry career, madoracle's Information Technology career and p00jkins' Factory career. And jphill11's Happily Unemployed and Self Employed careers, for sims who don't want regular jobs, such as business owners, artists, novellists and homemakers.
I rarely send sims to university, but when I do they generally study Biology, Psychology, Art or Drama, or one of my two custom majors.