Well, I like my Balao. It has extreme firepower, great radar, plenty of depth-keeping ability and a decent 20 knot surface ability. It also has a really cool shark mouth.
My S-Boat is growing on me, although it is nothing like the real thing. Captains just prayed to survive them to get into a real submarine. Ours work WAY too good. But they're a lot of fun in the game and are great underwater performers.