Ultimately the one that's loaded and in the hands of the most capable shooter.
Also it is almost impossible to find a weapon that would be universally "the most deadliest". For instance, the Davy Crockett, now out of service, being a recoilless rifle, could be considered a "gun". So could the M65 atomic cannon. However, if the enemy is within a kilometer or two, you would probably not want to use either. That would eliminate the shooter as well as the enemy, thereby severely limiting the weapon's future lethality.
Meanwhile any range of weapons, such as the MP7 or the Mag-Pul PDR are great for CQB work. But if the target is raining down artillery shells on you from 20km away, you'd be in trouble.
So you have to pick the right tool for the job.