Best tank gun of WW2?

Part of me wants to say the '77mm' - a handy concept that never really gained traction. Was it ever considered for Centurion or Black Prince, or was the 17-pr that much better?

 

Correct.

 

Does it have to be a conventional gun?

Can I put forward this little devastating monster:

Any German tank would get slapped about by it. It's also deadly to bunkers and buildings, and has the same effect as a 3-inch mortar agaisnt soft targets.

 

Oh dear! The Blacker Bombard. Range no more than 100 yards. Reportedly as likely to harm its crew as the enemy. I’ll leave others to decide if it was a worthwhile weapon.
Blacker Bombard - Wikipedia

 

Ahh yes, the silly myths about the Bombard.

First off Credentials:
Maybe of interest: Spigot books and talks!
I've only spent seven years looking at the Bombard (which is why I likely put it up in this thread, its been a big part of my recent life! and all my thinking devolves to it).

Range of the anti-tank rounds was 500 yards. However, the flight time of the projectile was very very very long. Thus the range of shots against moving targets, was suggested to be 150 yards, simply because longer range shots introduce too many variables for a good chance to hit. The rake sight was calibrated for 0, 10, 20 and 30mph speeds.
The whole bollocks about its lethality to the firer comes from one incident when the fuse was inserted the wrong way, just as the weapon was being introduced in to service, and the bomb detonated on the spigot. The board of inquiry wash eld the following day, and within a few weeks a Mk.2 fuse was produce which had metal flanges to prevent incorrect assembly.
I suppose the other problem is the possibility of breaking your neck if you have your head too close to it when it recoils. But the same applies if you have your head behind the breach of a normal gun. As long as you do not rest your head on the padded bar over the gunners sight you'll be fine.

 

I am awful at tank guns, but i will have a stab in the dark and say 17 pounder British

 

How about USN 5"/38?

On some of the early targets, we had to fire rapid continuous (about 20 rounds per gun per minute) fire for one, two and in one grueling demonstration, for six minutes at a time. Although our "doctrine" told us this could be uneconomical of projectiles (rapid "timed" fire was better) the "spot" from the NLO was "on" and the urgency in his voice conveyed to us a requirement for extreme performance. Later we went to continuous timed fire, more economical of projectiles and nearly as effective on a per unit time basis. The Army's Artillery General Officer ashore who was fighting this phase of his battle with our artillery, sent repeated messages of encouragement. Finally, in the waning hours of daylight, as we left the firing scene, he sent one of the most magnificent messages of appreciation to Rear Admiral Lyal Davidson that I have ever seen recorded. "Thank God for the fire of the blue-belly Navy ships. Probably could not have stuck out Blue and Yellow beaches. Brave fellows these; tell them so. General Lange." Later and without waste of language, he told vividly of tanks piled up in rubble and how attack after attack of the German forces had been blunted, and finally turned back, and the beachhead made secure. Again, by mail we received from this expressive and appreciative source, photographs showing the terrific damage inflicted on twelve German tanks. They were piled up like scrap iron. Many of us were truly amazed at the localization of effective blast damage from concentrated 5" 38 HC fire.

Tiger Tanks at Salerno, German tanks duel U.S. warships at Salerno in direct fire. 88mm supersonic gun fails to match Navy ships' rate of fire.

One part of the article could be posted on the 'Glaring Mistakes' thread though. I don't think anything in WWII was subsonic except mortars and some low powered pistols

We had been used to the fluttery sound of larger projectiles in arched trajectories. (Like our 5" 38s, most enemy artillery projectiles were subsonic.)