To find out which gun is the best in Call of Duty Black Ops 4 lets first discuss a few weapon classes that can be dismissed. The shotguns and sniper rifles can be fun to use but are too specialized to consider any of them to be the best gun in Call of Duty Black Ops 4.
Tactical rifles can be powerful in the right hands, but are limited in their usability to, for example, push a close range objective.
The best gun in Call of Duty Black Ops 4 must then either be an assault rifle or a submachine gun. Most of the time you will encounter enemies at medium and close range and both weapon classes have guns that are strong or good at either range. Submachine guns are more suited to close range engagements and flanking enemies to take them by surprise.
Assault rifles are better suited to medium range and taking on enemies head on in the thick of the fight.
The main weakness of submachine guns is that in competitive matches you will often face enemies behind cover that have entrenched themselves. You can take them on with submachine guns but you often have to spend a lot of time flanking with the risk of being spotted and taken out having wasted the time it took to flank.
With assault rifles you can simply shoot the enemies behind cover and push forward. Having established assault rifles as the source of the best gun in Call of Duty Black Ops 4 it is easy to pick the best gun. Of the assault rifles the Vapr-XKG with its minimal recoil and good damage stands out as the best. Therefore, the Vapr-XKG is the best gun in Call of Duty Black Ops 4.