A fighter bomber is a fighter that was regularly used for day-time ground attack and ground support missions with bombs, rockets and guns.
While also retaining the capacity to defend itself against enemy CAP fighters. which twin engine attack aircraft like the A26 or the Mosquito or Hs126 could not.
So we go with single engined fighter aircraft, I think! Noting as we do that almost all such s-e-fighters were used in this role or couldadopt it hwne needed, say over Normandy for the first three months
Noting that a GOOD or BEST one would have to be one that was mostly used for this job, because it was suited and/or proved so to be, even if not originally designed for it.
The A36 Invader version of the NA P51 was used as an attack aircraft and P51B and D units were encouraged to do frei-jagd and ground-strafing efforts on their way home from escort missions. But ground attack was very risky for the pilots as its glycol radiator was particularly vulnerable to LAAA fire - engine seizures within a minute or two far from home. And occupied Europe and Germany had heaps of LAAA by 44!
This seems to me to rule the P51 OUT of discussion, as one of the BEST fighter bombers!
IMO The WWII standouts were
UK/Cwlth built models - Hawker’s - Hurricane MkII, and IV, though to the end - and the Typhoon. Possibly/maybe??? the Spit5/7/8/9 LF versions where fitted with bombing shackles, being a bit less vulnerable? to ground fire than the p51.
Japan? WGAS!? eh?
Germany? the FW190,
USA built - P39 and P63, P40, and P47
The two Bell models were very effective in this role for the Russians.
the P40 had the longest service, and was a hugely effective jabo from NTH africa onwards.
And, it remained a tolerably capable fiVfighter type too - if the pilot knew how to fly it to its limits, which was do-able without massive muscular effort unlike say in a Zero, Me109, or Macchi. This also applies to the Hurricane and all the Merlin Spits.
Yet for me the record of the Jug as a fighter bomber from 1944 onwards is simply outstanding. And it has to be the winner! by ahair over the p40
Given the OP wanted a thread on the best American (made and flown by) fighter bomber of WWII it seems to me to have been an open and shut case!
JBTW I too dislike posts stating mere enthusiasm sans any cogent supporting arguments!
This thread was packed with them!
Tim B