I have no proof of any of this, just my own thoughts as someone being interested in flying etc. The 7 crew are: 1) Pilot, 2) Flt Engineer, 3)Navigator, 4)Wireless Op, 5) Bomb Aimer/Front Gunner, 6) Mid Upper Gunner, 7)Tail Gunner (Arse End Charlie). As attacks normally came from the rear, 7's got hit a lot. Fighters also tried to take out the pilot. As he(1) and 2,3,4 all sit close together, I think they would not be too likely to survive. This leaves 5 & 6. Frontal attacks could take out the bomb aimer, but he had the advantage of laying on one of the escape hatches, so it was easy for him to get out. The Mid Up gunner was isolated between the two ends, so was less likely to get shot, but had to climb out of the turret, get his parachute(no room to wear it in the turret) and make his way to an escape hatch, whilst the aircraft was probably spinning and/or burning. My guess would be the Bomb Aimer, but like I said, I have no proof.