Before starting, I shall note that Germany and Japan both had developed very advanced designs late in the war (Type XXI/XXIII and I201/Ha201 respectively) which quite outclassed all other subs. However, as has been noted, none of these vessels became fully operational, so I shall exclude them for the moment. The Type XXI in particular combined not only an advanced streamlined form, but also improved periscopes/sensor masts, improved battery capacity, schnorkel, and an advanced conformal passive array integrated into the fire control system – all the elements of the post-war sub up until USS Nautilus.

The subs I shall consider are the following:

USA – Fleet boats

Japan – the various large I-boats

Britain- The S, T and U classes

Germany – the Type VII and Type IX boats

Also, one cannot totally ignore the qualities of submarine weapons (particularly the torpedo) in considering a submarine as a weapon system.

Italy, France and the Soviet Union also produced serviceable designs with their own virtues and deficiencies, but in general I consider them to be behind the four other naval powers in terms of efficiency. Some may dispute this, but I would cite mediocre diving performance, surface speed and range, endurance and depth performance in the vessels of these countries without any particular offsetting features. Also, their torpedoes varied from being adequate (but not outstanding) in reliability and performance to being largely useless (e.g., the small French 15.7″ and Italian 17.7″ weapons).

USA – The Fleet boats in my opinion contained few superlatives, but did benefit from a synthesis of qualities that made them quite suitable for the war they were called to fight. Their best qualities were probably a large torpedo capacity and a long endurance. They also possessed good surface speed, adequate diving depth, a torpedo firing computer, the usual US mechanical reliability in all but a few, and adequate sensors (no sub before the Type XXI had really good sensors). Their worst features were that they were comparatively slow-diving and relatively unhandy submerged, which could have been fatal had they fought the same war that German and British subs fought. Also, the scandalous performance of US torpedoes early in the war all but negated any virtues of US subs and their crews until corrected by mid-1943.

Nonetheless, the Fleet boats went on to fight what was in the final analysis the most effective “guerre de course” in history, although I would argue that the success of that campaign had more to do with good strategic decisions, operational employment and crew skill on the part of the USN and similar errors or deficiencies on the part of the IJN. I regard the Fleet boats as good, serviceable designs which did not in any way by their deficiencies limit the American ability to wage its war against Japan, but neither do I think they brought any special qualities that gave the Americans any particular, disproportionate advantage. Once Fleet boats received adequate weapons, they proved completely suitable platforms, and as such must be well-regarded.

Japan – The Japanese I-boats reflected a mistaken operational concept on the part of the IJN with regard to submarine warfare, and as such were quite poorly suited to the war they were called to fight. As an extension of the surface fleet, they possessed excellent range, very high surface speed, a powerful gun armament, adequate weapons load, good communications capability, and many even carried an aircraft. Like the US Fleet boats, they were capable of extended operations far from base; to an even greater degree than the Fleet boats, their optimization for the reconnaissance role impacted their general submarine qualities and made them quite vulnerable to USN countermeasures. Over-emphasis on this erroneous operational concept eventually produced monstrosities of very dubious value.

The I-boat’s greatest virtue was not its own, but resided in its excellent torpedoes – probably the best of the war. These weapons had exceptional speed and range, and had the virtue of leaving less of a wake than comparable compressed-air weapons. All-in-all, the I-boats could be considered as largely a failure for the war they were asked to fight, even though the aforementioned designs introduced at the end of the war proved that the Japanese were quite capable of designing excellent craft, if starting from the proper operational concept.

Britain – British subs may well have had the most varied war of any, in that they operated in the anti-shipping role in the Mediterranean, as reconnaissance/special operations platforms in the North Atlantic, and on operations similar to that of the US Fleet boats in the Pacific. In that regard, they were called on to exhibit the full range of submarine virtues.

British submarine development had experienced teething troubles in the 1920s-1930s, but by 1939 most of these had been worked out in three good designs. The S class was a good general-purpose design offering adequate (but not outstanding) performance in all sub qualities. Compared to the US Fleet boats, they had not the range and endurance or weapons capacity, nor the surface speed, but they were quite a bit more handy and also fairly quick-diving (in this regard only the German subs surpassed the British) – qualities which meant the difference not only between success and failure but life and death in the North Atlantic and Mediterranean. They were useful in all theatres, although they did not equal the performance of the US in the Mediterranean nor of the Ts in the Far East.

The U class exhibited outstanding qualities in terms of diving speed and agility, surpassing all but perhaps the Type VII in that regard. To do so, it made sacrifices in all other submarine qualities, and as such they were special-purpose vessels really useful only in restricted waters operating fairly close to base. They came into their own in the Mediterranean and were extremely successful there.

The T class most closely approximated to the US Fleet boats of all the classes considered, and this similarity became ever more pronounced as both were modernised in the 1950s. As such, the Ts shared many of the virtues and deficiencies of the Fleet Boats – although neither to the same degree. They had not quite the weapons capacity, surface speed and range of the USN boats, but also were more handy and quicker-diving. In the Far East they ended up specialising in missions at heavily traversed choke points where their somewhat better handiness benefitted them.

The British Mk8 torpedo was an excellent weapon – perhaps second only to the truly exceptional Japanese weapons. While it suffered some of the same problems with the magnetic pistol suffered by US and German weapons, these were promptly resolved and it went on to be a reliable torpedo of excellent performance and good lethality.

Germany – German submarines enjoy a very high reputation, and in many ways deservedly so. The Type VII was a superb submarine for the European war initially envisioned, and enjoyed truly exceptional diving speed, underwater manoeuvrability and diving depth – in each category probably unsurpassed by any other. For extended operations further from base, the Type VII was barely adequate due to its limited dimensions – nonetheless, they conducted missions far in excess of what might have been expected pre-war. The Type IX had better qualities for extended operations than the Type VIIs, but still were not fully adequate. While they were considered “cows” by German crews when compared to the Type VIIs, they were still quite handy compared to Allied subs – and again very deep-diving.

The Germans attempted to improve the weapons capacity and range of both the Type VII and Type IX while extending further their already excellent depth performance, but could not fully overcome the inherent limitations of the designs. To compensate, they developed a series of specialty subs (the Milch Cows) to replenish the attack boats, but this became a weak link in the system. Also, the Germans continued to introduce into their sub force advanced ideas at a rate probably unequalled by any other country: the revolutionary schnorkel, radar warning receivers, homing and pattern-running torpedoes, etc. Finally, due to their tight control of their subs, the Germans were able to concentrate their subs effectively into wolfpacks, but they also increased the vulnerability of their force to HF/DF.

The G7 torpedo experienced teething troubles at the start of the war exceeded in seriousness only by those of the US Mk14, but the Germans quickly eradicated these. The weapon went on to establish an excellent reputation, and in its electric G7e version was paid the ultimate compliment: Westinghouse reverse-engineered a captured G7e, and it was adopted by the USN as the Mk18 in preference to their own MkII.

In summary, I would have to consider German submarines as perhaps the best, in that they possessed certain outstanding virtues which enhanced measurably their effectiveness for the war they fought. Also, they introduced a number of advanced features which would become standard in later subs of all nations. Paradoxically, they also possessed deficiencies which limited measurably their ability to prosecute the war they were called to fight.

By comparison, the war of the US Fleet boats lent itself well to their virtues while making little demand on their worst qualities. I can see why one might regard the Fleet boats as the best WWII subs as a consequence. As such, they proved perfectly adequate to their purpose, although under different circumstances (Mediterranean operations, for example) they might have shown up poorly compared to German or British subs.

Compared to USN design practise, the RN developed a specialised sub for restricted-waters operations (the U class), a specialised long-range sub (the T class), and a general purpose sub useful for both. The S and T class had good all-round qualities and were better-balanced than German, US or Japanese subs, but lacking some of the superlatives of the US and German vessels. The U class was optimised for such operations as North African traffic interdiction out of Malta, but otherwise was too specialised for general operations.

Japanese subs, while outstanding technical achievements, suffered from an erroneous operational concept and were overall poorly designed for offensive Pacific operations. Because of the excellence of their primary weapon, they achieved results disproportionate with their own intrinsic qualities.

Honourable mention must be given here to the submarines of the Netherlands. They were stoutly built, with good surface and underwater performance, good endurance, good sensors and good weapons. They were ahead of their time in terms of streamlining and, of course, they invented the forerunner of the schnorkel. In 1940, overall they were as good as any in the world.