Engineer. Mainly because it's what I enjoy doing personally (though I'm taking accounting--better job opportunities here in the sticks). I simply enjoy the fact that making gadgets is part of the job. Scientist is a close second. Mainly due to all the gadgets that can be made in it.
Due to the game's rating and the whole idea behind the Sims, we can't have severe, time-sensitive trauma/injuries, Sims dying if something isn't done, or chronic diseases, which takes all the realism out of the doctor career. It feels like it might as well be the "doctor's office" career; given all Sims who show up presenting illness are nothing more than minorly ill. I don't see why injuries can't be added to mix things up. Same with the police career and anything other than presumed robberies. Violent crime doesn't do well with the whole Sims series either.