I'll leap into this often brought up and controversial subject with a quick foray.

It is generally conceded that the German XXI 'elektroboat' was significantly more advanced than any other sub design in the world, and I agree.

But the XXI never saw combat service. So does it count as a WWII sub?

If you say no, among subs that actually saw combat, I'll vote for the late war 'thick skinned' US fleet boats, edging out, say, a late war schnorkel equipped Type VII.

Compared to late war German boats, the US boats had superior range, endurance, habitability, greater firepower (no. of torpedoes, tubes etc) and, most importantly of all, superior electronics; secure encoding machines, good radar, good radar warning receivers, etc. They didn't have the crude schnorkels of later German U-boats, but with good radar this was not as much a handicap. They could not dive as deep, but both could go well over 500 ft, deep enough to get below the thermocline, at least in the pacific.

Actually a good comparison is very difficult and complex, and hard to do on an apples to apples basis.

No US boat would have likely fared very well, or any better than the contemporary German boats, against the massive allied ASW of the later war years.