*sigh* The term "best" is so ever used it is a bit of a shame.
No tank or tank gun in WWII was absolute "best". There are far too many factors to take in to consideration. Be sure and specify what category the term "best" is being applied to. The German 128mm may have had the "best" AP ability, but it would be quite low on my list of "most desirable" main guns for a tank in WWII.
Take in to consideration sights, range, AP ability, HE ability, reload rate, maintenance costs, manufacturing costs, etc, etc. Even at the end of the war, the German Panzer IV H with its 7.5 cm KwK 40 L/48 main gun was still highly formidable. One has to wonder what would have happened of the Germans would have upgraded the sights of the H and allowed it to engage enemy tanks starting at 1500m. The Panzer IV H was lacking only in armor, but it was still average.