What make the A-20G the best addition? I can think of 7 other aircraft that do essentially the same job, and 5 of those do it better IMHO.
The P-47D-25 and 30: more guns, 2,500lbs of bombs and 10 rockets, faster, better climb rate and better diving.
The P-38L: More ammo, 2,000bs of bombs and 10 rockets, faster, better climb rate, better diving and better turning.
The Mosquito Mk VI: 2,000lbs of bombs or 1,000lbs of bombs and 8 rockets, 4 Hispano Mk II 20mm cannon with lots of ammo, faster, better climb rate, better diving and better turning.
The Bf110G-2: 1,000kg of bombs, 4 210mm rockets, 2-4 20mm cannon, 2 30mm cannon, faster, better climb rate, better diving and better turning.
The Typhoon Mk Ib: 2,000lbs of bombs or 8 rockets, 4 Hispano Mk II 20mm cannon with lots of ammo, much faster, better climb rate, better diving and better turning.
Don't get me wrong, the A-20G is a great addition, I just don't see what makes it best.
(Bf110C-4b and Il-2M are the other two aircraft that do the same job, but not as well as the A-20G)