Sorry to ask this really dumb question. Which Zero would you say has the most lethal gun package?
Thanks,
Slade 
A6M5b without question. The Type 99 Model IIs it carries are the second hardest hitting 20mm cannons in the game and it replaces one of the 7.7mm machine guns with a 13.2mm machine gun.
The Type 99 Model IIs also have more ammo due to a belt feed and a much improved muzzle velocity. Their weakness is their horrible rate of fire, much lower than any other 20mm cannon, even the Hispano Mk II's. That said, it is not nearly low enough for the Type 99 Model Is on the A6M2 and A6M3 to compete. The N1K2-J carries four of the Type 99 Model IIs with a boatload of ammo, really making the gun shine.
All that said, I find the A6M3 the most fun to fly and fight in of the A6Ms.