Like others have said, it depends on the situation. I like the convoy attacks where you repeatedly attack the convoy. If you are very patient, every one of your torpedoes can hit a ship and you can come back to base alive.
I think something that plagues novice players is they see all these ships and feel it would be a 'waste' to leave empty handed. However if you are good at plotting the escort's course you can simply break off the attack, sprint ahead, and make another attempt. Catching the convoy far enough out to sea allows you as many attempts as your torpedoes and fuel will allow, if you are PATIENT enough. For example:
There was one convoy I attacked where during each approach, an escort spotted me. One of the nice things about escorts attacking you, as opposed to any random DD puttering out in the ocean is that they are obligated to stick with the convoy, and generally will break off their attack to catch up with the rest of the convoy. I took advantage of this. Every time I was spotted, I'd throw a decoy at about 50m depth, dive to 120m and creep away at 1kt. After they disappeared off the horizon I tried again. On my fourth attempt I was able to sneak into the convoy and sink a T3 cargo, a T2 tanker, and a cruise liner with my 5 torpedoes
which was definitely a reward for my patience.