Infantry
Rifle grenade No 68 - best penetration, compact, no backblast, not as obvious as the PIAT & Bazooka.
Vehicle mount
Er, what type of vehicle?
The 32 pounder on the Tortoise heavy SPG - would be the most frightening.

Though not strictly used in combat.
Artillery
17 pdr - RoF, penetration, weight - just a beut!

Though the 6pdr firing APDS was also amazing.
Aircraft
57mm of the Mossie - recoil, penetration, accuracy(PlanD?), ability to damage.