One of the actual best guns in the game for raw DPS is the R99, but I bet you it isn't taken that often by the majority of players. It fires very fast and thus requires a lot of tracking skill to make every bullet count.
No, OP is wrong about everything. Peacekeeper is likely the number one most lethal gun in the game right now. OP is a Peacekeeper abuser as you can see from his other posts if you've watched him around the forums. He is deathly afraid of his crutch being taken away, lol. People do not use the most "potentially" strong guns in any game, they use what has the best mix of low skill ceiling to high power. This is why GOATS comp has been ruining Overwatch for the better part of a full calendar year. GOATS comp is that game's version of the Peacekeeper...extremely user friendly and yields amazing results. That doesn't make it a good thing for the game and Blizzard has tried loads of things to stop the abuse of GOATS in Overwatch, yet it's still very strong and is still being picked heavily in their second season of Overwatch League.
But Blizzard is dealing with a 6 hero team comp and trying to balance it around the other 23 (soon to be 24) heroes in the game. All Respawn has to do is nerf the range and fire rate of the Peacekeeper. Lucky us. That nerf is coming and the game will be better off.
