(generally picking a country's highest-production aircraft not already in the game)
B6N (Most-produced Japanese torpedo bomber, taking over for the early-war B5N)
Most produced isn't always the best route. Many times planes are made in large numbers but still have very small impact on the war.
One example is the P-40N. The most produced P-40 variant, but of the 5000 or so built, most were state-side trainers for the US. They served in foreign nations more than they did in the US forces! RNZAF, RAAF, some USAAF, but the vast majority were trainers in the U.S.A. edit: Silly me, how could I neglect to mention VVS exports? /edit
Another example is your B6N. While it was intended to replace the B5N it never really did. It had a small initial debut with about 100 planes right about the time US forces were massing in Bougainville area. They were readily wiped out in the small numbers they were available, and removed from frontline duty. The IJN carriers were sunk and the remainder were all based in Japan with no operational use and little combat records to review. Of the 1200 built, only about 150 or so saw real combat.
So in summary, production numbers don't tell the whole story.