I have two:
BE3x --> BF1x The area between Ireland, Great Britain and France where convoys regularly show up just on the line where it gets real deep... I decided to never again make an attack run in shallow water again
the other is also at Gibraltar... it is like a funnel where the convoys have to go throu ... and I am waiting for them like a funnel spider in her hideout
though it gets tricky... that prey starts getting theeth and claws and fights back ('end '41 early '42)
depends on where I had made my patrol and how many torps are left etc.
Freetown is also a good hunting ground as there seems to be less chance backup appears but it is just too far for my VII
Good huntnig and fat prey - Gute Jagd und fette Beute ihr Himmelhunde
(I cannot translate Himmelhund
)