I really enjoy the P-51B
Its quick, fairly nimble and can hang with the Zekes, La7s and most of the Spits if you fly with careful flap control.
Having only 4 .50s means you realllllllyyy have to hammer away to get a kill. And with a small ammo loadout, accuracy is important.
Unlike most of the planes in the game...when I see a 51B driver landing some kills, I'm quick to that fella. Those are hard earned.