I thought it would be worth sharing our favoured loadouts for MvM, mostly because i just creamed a session using the KGB and wanted to share it's OP-ness with the world. KGB > +4 Health slots on kill, + 4 attack speed and +3 run speed so you don't get kited. Once you get the first kill it's just 1 shotting anything except heavies and bosses, but usually you've racked up enough crit time to take down even boss bots in 5-6 hits. Another benefit is upgrading the attack speed is also great for spamming the tank without ammo. Not sure what the DPS works out as but it's pretty wicked with +4 attack speed and 5 seconds of personal crits. One more benefit is because it's melee you tend to just hoover up the money as well, most rounds we got credit bonuses for acquiring every credit. The only downside i see so far is it makes you weak against pyros which is mostly situational (1: if there are pyros at all and 2: if they aggro onto you), but the upgrades are so cheap for melee that eventually you can throw some points into your primary weapon by the third wave.