Scout - Sandman, you trade survivability for an underpowered mechanic.
Soldier - Righteous Bison, no explaining needed.
Pyro - Gas Passer, super underpowered, only useful in MvM.
Demoman - Ullapool Caber, one of my favourite melees for only the reason of one shotting stuff on mini crit, still pretty bad.
Engineer - Pomson, pretty useful to deal with dead ringer spies, but not much else.
Heavy - Buffalo Steak Sandwich, only ever paired with Fist of Steel, Warrior's Spirit and Holiday Punch for taunt kills.
Medic - Vita Saw, lowers health for a chance of saving uber on death, which is ultimately overshadowed by the ubersaw.
Sniper - Tribalman's Shiv, great for tracking spies but not anything else, also experienced spies tend to shoot snipers if they approach them so thats kinda negated too.
Spy - Enforcer, negating resistances upside very rarely comes into play, as for the additional damage, only use would be 2 shotting snipers from up close, but the amby already does that from an even longer range.