1. I can say the Ki-43 was the best aircraft. It out turned it's enemies and was very light allowing it to climb fast. There are several variants of it, the best i think is the one armed with the two 12.7mm machine guns. I read somewhere that at least one had a gun of a strange caliber. It was like 23mm or 27mm i think.
2 Now that I'm blabbering I'd like to mention an account where a ki-27 shot down at least one p-40. I also read about a biplane warding off superior and numerous enemies on it's own during the war. These two pieces of evidence is sufficient in that we can acquiesce that experience was the major factor that made any craft superb during the war. I agree that the question is too bland.
1. Best of the war, questionable IMO, though I doubt there is any single 'best', not sure what it even means. The best a/c from purely technical viewpoint were 1945 types, naturally, but many earlier types had good success in their own time.
The Type 1 Fighter (as Japanese Army operating units almost invariably referred to it, later codenamed Oscar, operating units almost never used the Kitai numbers, eg. Ki-43) had considerably success early in the Pacific War. At that time Allied units didn't distinguish it at all from the Navy's Zero; later references to Allied success v the Type 1 are mainly from mid-1943 onward. In early 1942 the Type 1 achieved a several to one kill ratio (based on each side's loss accounts, not relying on claims) against Brit/CW/Dutch Buffaloes and Hurricanes, although it didn't do as well against AVG P-40's in fewer engagements. And the Type 1 in reality held its own against USN/USMC and USAAF a/c in the Solomons in early '43 and in New Guinea from late '42 respectively, again until well into 1943. The JAAF did really like the Type 1's manueverability, even when the Type 3 ('Tony') first came along in New Guinea, contrary to some Western accounts saying the Type 3 was immediately preferred. Experienced Type 1 pilots felt they could not be defeated by 'boom and zoom' tactics because they'd see the Allied fighter coming and turn into its attack, and sooner or later the Allied fighter would have to slow down and come into their preferred element (eg. how the US P-47 ace Neal Kirby was shot down in 1943). Now, did this always pan out? no, eventually Type 1's were typically bested by Allied fighters, but it performed well to at least fairly well in late 1941-mid '43.
Most Type 1's encountered until mid '43 had a single 12.7mm and a single 7.7mm. Francillon's description of Ki-43-I a, b and c with 2*7.7, one each, and 2*12.7 respectively isn't correct, almost all -I's had the mixed armament.
2. Type 97's ('Nate', Ki-27) shot down a number of P-40's. The AVG mostly met Type 97's and downed around 35 of them (per Japanese accounts) to around a dozen P-40's downed by Type 97's (AVG's accounts). However that lack of Type 97 success was the exception. Against the USAAF in the Philippines and DEI in 1941-42 Type 97's downed around 6 US fighters (P-35/P-40) and while the exact Type 97 losses aren't known in that case the P-40's didn't even claim 6 Type 97's. Likewise v the Hurricane in early months of the war the Type 97 downed 8 for the loss of 5, and v the Buffalo downed over a dozen for the loss of 1 or 2. IOW except against the AVG, the Type 97 units were competitive with the Allied opposition they encountered early in the war, and by mid '42 it was mainly phased out of front line service, though there were some later encounters (eg. P-38's engaged some Thai AF Type 97's in '44, USN F6F's encountered some Japanese ones in '44 also).
Joe