For the best medium AA mounting on a warship my vote goes for the 40mm bofors. A Swedish design, used by the US, British, German, and Japanese navies, (and versions used by these countries ground forces). It was reliable, had a heavy weight shell, a high rate of fire (120 rds per minute, per barrel) and when powered and tied to the Mk 51 director was extremely effective. 40mm US quad mounting. Mk 51 director. Any other contestants?