Pilot

Pro: Glamor, half-decent living conditions at airfield

Con: Very High casualty rate (operationally as well as combat.)

Tank Crew

Pro: none?

Con: Early tank interiors loud enough to cause hearing loss and tanks overall very uncomfortable. Tanks allocated only to major attacks and therefore high casualty rates expected. Tanks break down a lot, and then you're just infantry in a very hot part of the front.

Trenches (presumably infantry?)

Pro: More likely to be wounded or sent home due to illness than killed outright. Chance of going to a quiet sector for long periods.

Con: Terrible living conditions, Mud, no sleep and short rations due to shelling interrupting logistics. Since the vast majority of combat soldiers were infantry, the death rate is about the median. Exposure to shelling causes psychological issues that linger.

Ship

Pro: Good living conditions aboard ship. Low casualty rate, especially if on a German battleship. Combat is rather rare for any given ship. Con: Some ships safer than others High casualty rates amongst U-boats.

The Navy offers the best chance of survival and not totally horrible living conditions, so I'd pick that.