Ideally - you'd want the door to be between the landing gear, so as to prevent the first pallet put in the door causing the tail (or nose) to flop on the ground.
But there are always complications. In the case of the 777F, putting the door (for a standard container size) ahead of the wing far enough to minimize risk of a loader running into the #1 engine would have put the door into the area where the fuselage is shrinking down to cockpit diameter.
So Boeing put it behind the wing (as close as possible).
The 777F and that Rear Cargo Door | Things With Wings
On a C-47 taildragger, behind the wing is a no-brainer: 1) away from the props, 2) lots more area, 3) closer to the ground, 4) between the main gear and tail wheel for solid balance.
https://www.army.mil/e2/c/-images/20...-14-152855.jpg
On stretched airframes, and especially tail-holers (727/MD-80 variants) - doors ahead of the wings are 1) safely between the gear, 2) in the largest (and often "only") available area, 3) away from the engines.
http://cargofacts.com/wp-content/upl...thcaption1.jpg
That West Air ATPF looks a bit tippy to me, but again - 1) away from props, 2) largest area - plus the forward weight of the engines may be sufficient counterweight, for a careful loadmaster using a calculator.
