Unprofessional opinion mode:
There are planes that are better fast:
P51
F4U
Fw190
La5FN
There are planes that are better slow:
Bf109
Spitfire
There are planes that play either way, but are also limited in some manner at either extreme:
N1K2J
C205V
Personal Takes:
P51
Best bet is to use its speed and zoom to gain and keep alt advantage. In a furball keep it fast and extend (fancy word for "run screaming like a girl") when cons start dropping on your 6. It rolls well at speed, and this can be used against planes that get sloppy at high speed. Keep fight vertical as long as possible, then extend when advantage is lost. 6x50 hurts!
F4U
Very similar to P51, though it rolls faster at speed and climbs slowly. Excellent zoom characteristics, so go for the high speed pass with a low-G climb for alt. Slow accleration means death to an F4U caught low and slow.
Fw190
Best at speed, the 190 can give a good account of itself due to hard-hitting guns and great roll-rate and great initial turn speed. Every effort should be made to keep speed up, as once slowed the turnrate is galacial. This one is used best in slashing attacks and quick egress.
La5FN
The La5FN can easily handle any of the above mentioned aircraft with ease at their own game, it's sole limitation being the relatively light armament. The 2 cannons are good, but you need to be close. An La5FN on your tail is a Mustang's nightmare.
Bf109-G10
Nothing accelerates faster, which is why it's a good strategy to slow a fight down with most opponents. Once their energy is bled away you can climb above them and kill them. Be wary of control lock-up at speed, because it can get out of control in a hurry. Diving a 109 for long is therefore a bad idea. The worst thing about the 109 is the light ammo load and range.
Spitfire MkIX
As near a training craft as one can get in AH. It has good guns, good speed, nothing outturns it (though C205V is close), climbs great... it is probably the easiest plane in which to be successful. Light fuel load hampers it somewhat, and blackouts occur very quickly, but when flown properly it is a tough bird to beat.
N1K2J
Easily the greatest firepower carried in the game, combined with a good climb and turn performance. It is a little slow, so you don't get to choose when the fight begins or ends in most cases. Anyone opting for an HO against a Nikki asking for it.
C205V
A very balanced plane. It does nothing to the extreme, but is not bad at anything. You can fly this one either fast or slow and do well. It sports a good punch and carries a load of ammo. It handles well at any altitude.