in my personal experience
Fighters:
no way a MN ftr is best than KM or RM...
besides at low tier (T1-Locals > T2 > T3) km works best than any other nation... (of course US T4 cripple every other nation)
Fighers by nation: US > KM = RM > MN > IJN > SN > UK
IJN will look crappy, but i have faced lvl 120 overvetted IJN Ftrs that can work real nice... but the cost to get there is quite high
SN and UK are not usable... they can't catch T5 scouts almost anytime, still SN are better than the crappy UK
Dive Bombers:
SN are quite fun and they can take a lot of punishment, IJN are almost as good, but they can't take any damage, wich they compensate with speed
Don't bother about nation's bonus... they will all cap at Lvl 85-90 (assuming +12 BVE)
DB by nation: SN > IJN > MN > KM (good damage low fuel) > USN > RM > UK
US is the last usable DB, they have average dmg but nice speed, besides they look like T2 ftrs lol
RM and UK are not ment to be used... just go for TB if you want to do nice damage
Torpedo Bombers:
UK finally don't suck at something... those TB are a boost, RM have more damage than US but the torp speed sucks and many targets will dodge your salvo
like DB, TB will cap at low lvl... 85-90 for any +12 BVE so don't think about nation bonus
TB by nation: UK > RM >= US > IJN = MN (quite usable due to high speed in fact) > KM > SN
RM extra damage compensates with better torpspeed by US
MN and IJN are quite usable even for TBW configurations as they have high speed TB and high speed torps so they compensate with the dmg loss compared to best nations
KM and SN are not usable at all, KM besides low dmg have range issues, but for hunting SS they are still better than crappy SN