V&W, River, Black Swan. They are common in groups of 2 or above and they cannot be destroyed in a gun duel. They are too strong and they have got too much HPs. A fish is necessary

I could destroy a Flower ,in case of emergency and my willing to sacrifice your seamens and HP to it with a 8.8 cm naval gun

Yes, I agree that battleships are large threat, but they cannot do anything (except ramming you and destroying your desk) after you have got to periscope depth

For other, such as Fletcher or Tribal, they are uncommon and they can be neutralized by a fish also

Anyway, never attempt to sneak into groups of destroyers consisting more than 3 DDs. They can work very well, with one dropping DCs while others tracking your position!!!

Anything Corvette and above is dangerous including the mercs when they are armed if you are surfaced
Below water I find the Destroyers are the most dangerous then Frigates then Corvettes
The patrol boats arent that deadly but they have a tendancy to call in help - usually in the shape of destroyers

My last death was due mainly to a Flower corvette after attacking a convoy near Gibraltar and trailing a wounded cargo ship. After running out of depth charges, it continued making runs at me. I was going silent slow and around 150m deep up and down, but it was somehow always pinging me again (weather was superb and calm, which probably didn't help me.) After 1 hour of the game, the trailing escort from the convoy which was by that time far away, left the convoy to join the Flower. As soon as they were both on spot, the Flower would remain stopped or at slow speed using the Asdic while the destroyer was making precise attack runs. It lasted another 45 mns or so, and I guess thru loss of concentration, I turned on the wrong side during one of the runs and got hit on the bow (bow torpedo room damaged) I didn't have time to order going up to a safer depth that the lights in the command room started blinking, water to leak in and black screen. Back to Sept 39 once more...

For GWX....ALL ships late war ;)