I too love raiding enemy ports once in a while!
If you are feeling "adventurous" (or maybe suicidal haha), i suggest you try the ports of Scapa Flow, and Gibraltar.
Ibe sunk a few battleships on my "visits" to those ports.
Be adviced tho, Gibraltar has a shallow sub net on its entry (at least in GWX), so you have to go surfaced or under it.
Another tip i can give you is, when you are planning on attacking ports, try to coordinate your "visit" so you arrive just after nightfall. That way you have all night to infiltrate, find targets and escape.
good luck!