My sub of choice lately has been the type VIIC, I normally patrol round the denmark straight's southern entrance near to the "Milch cow"(forget the name at the moment) giving me some flexability to head states side or back south.
Looking forward to giving a IXC a blast and hunting in the warmer waters of the carribean..tankers galore in that region
Overall I would rate
Type VII as fave-She got my first 200K worth of shipping
IX-Have sailed one a while ago,was good slow dive times though
Type II-Sailed briefly,hasn't got the range or weapons capacity I enjoy
XXI-Sailed once,used a mod to set up a hypothetical career...sank a grand total of 2 ships for 10k before being sunk myself. Disaster in shallow waters,will have another go in the deeper water at some point,before passing a final judgement