Active/Playable Careers

And for those not averse to using mods (gee, if only I had a dollar/euro/pound/simoleon every time I started a sentence this way), Zerbu's Ultimate Career mod allows many of the rabbithole careers and schools to become open. With some Do It Yourself design, sims can choose between going into the rabbithole to work or using the skill specific objects the player provides on the lot to advance through their careers. :)

I usually do the firefighter the most.

I do enjoy the Architecture Career, but my car kept getting stuck in my inventory everytime I drove to a location.
And the career is somewhat glitchy.

I really like the medical career. Even though your sim spends a lot of time in a rabbithole in the beginning you later get the opportunity to work outside the hospital as well like running a health clinic, vaccinate sims, make house calls etc.

The stylist career and the architecture career are also fun. But I would suggest not to have a car if you choose architecture because, like GraceyManor mentioned, it always gets stuck in inventory unfortunately when you drive to a client.

I have played the ghosthunter career several times and had a great time! It comes with Ambitions and has lots to do. My Sig picture shows the updated ghost catcher you finally get. But be careful playing that career in Bridgeport. The ghosts tend to get stuck in the walls. A real bummer because traveling around the city catching ghosts was quite a lot of fun and let you see all of those apartments.

I like the singing career best. I love to hear my sims sing, but especially singing duets with the karaoke machine, with level 3 skill, of course. It's kind of painful to listen to sims with no skill sing karaoke.

I didn't even know TS3 had active careers. I am so exited for this game. :D

I like a lot of the careers: Ghost Hunter, Firefighter, the showtime careers (magician being my favorite) and I loved my sims to do gigs as a band.

Have you tried all of them yet? That's what I would suggest... the one you haven't done before. :)