Of course, it depends what I get. Also, it depends on my ship. For the II, watchman comes first. Second priority is the sonar dude, then a repair officer side-by-side with engine qualifs.
No torpedomen are needed, nor any gunners, because of the few fish and lack of survivability. In '39-'40, you can take the aircraft on yourself, better than most AI.
With larger boats, say the IX, the difference makes sense; Sonar, weapons, engines, repair officer, watchman. It's also a bit of a matter of playstyle, if you prefer surfaced or the bottom of the ocean - patrolling large areas or keeping relatively still in one place.