My personal favorite is the Type XXIII. Small, fast, very quick dives, with a limited torpedo load to make things challenging. Just the thing for harbor raiding, or perhaps layin' a smackdown in late '44 or early '45.
The one big problem: We don't have a playable XXIII
.
At least, I haven't seen one available. I do know someone was working on it, but I keep checking for it and it isn't around.
I'd prefer having the XXIII to the XXI anyway: Only one or two XXI's actually went on a war patrol, too late to sink anything.
Several XXIII's made patrols, sinking 5 ships. The last two ships sunk by a U-boat in WWII were sink inside the Firth of Forth by a Type XXIII (same boat: Two torps, two kills).
