Assuming that every Merc has all the stats of all the weapons but somehow in a real life situation, this huge chunk of text should help you decide who would be best. I will be talking about stock weapons over anything else to keep this a little shorter.
The Scout's Ability to run fast gives him the advantage of dodging bullets and being able to get intelligence (No pun intended)
His double jump would be almost like a very high jump, also good for dodging and going places.
Scouts speed and mobility in real life would be of very high importance to helping the side he is on, his weapons are pretty normal; A 6-Shot Scattergun that can put a huge hole in somebody's abdomen in seconds, a Pistol being a pretty basic yet effective item for a fight, and a bat.
Overall the Scout is good for doing what he does in comp games, recon work for the team and small kills every now and again.
The Soldier, A monster to fight.
With his likeliness of being fine from a shot if he can recover being higher than all but one, he can roll back out onto the battlefield in a short amount of time.
His rocket launcher is already just... Well it's a rocket launcher. Do I need to explain it?
A Shotgun is a great Damaging tool in real life, a Short Range gun to go sweetly with his Long Range gun.
Just like the Scout his melee is just a dumb weapon, or is it!? What if he was digging a trench? It's already useful for non combat situations.
Overall the Soldier is just going to run onto the front lines and die.
Hahaha, the Fire Man!
WM1.
That is all.
Demolitions Man.
A big drunk Scottish Cyclops, possibly the best weapons in the worst hands. With his drunkenness he is detrimental to himself and others, with team killing being more than likely and missing being extremely likely with his lack of an eye, he's not a great choice.
The Grenade Launcher can blow up Buildings and people.
Sticky Launcher is even better for those things but with a remote explosion device is even better.
What even is Demoman's Stock melee? A bottle? He's not hitting people with that, he's drinking from it!
The Demoman is just going to blow everything up, he doesn't care.
Heavy, that loveable lunatic.
This man is a rock. With a minigun. He's like Dwayne Johnson in any of his movies, lives through everything, gets money like crazy, and kills anyone who needs to die unless he's giving sandwiches to little girls.
He has a freakin' minigun! How can you beat that!?
Shotgun, huh? Check Soldier.
Fists. He will fist you. In the non-sexual way.
Overall, "HOLY CRAP HE'S GOT A GUN! GET AWAY!"
Engineer. This dude can build a Heavy/Soldier combo using a Wrench, do you know how cool that is!? He can supply ammo and healing to the Army.
His Shotgun is on par with the others, glack glack glack glack glack glow.
A Pistol can kill a dude with a good shot.
That wrench is his real weapon, making turrets and crap.
Summation of the Texan? He has guns.
It's like Sniper now, right?
This New Zealand born, Australia raised croc hunter would get set up on a high ledge or an elevated hill with a scope and get left alone for a few days.
His rifle speaks for itself, you wanna know what it says? "BLAM!"
He won't need an SMG, but it's there.
Oh my gosh the Kukri is cool, he could sit really far back and snipe dudes, also sniping food, cutting it up.
He's fine. Sniper don't care, he's cool.
(Voice: MEDIC!) idk he's probably fine if he sits out back and heals people.
Syringe gun can be used as a poison injector or something or just shoot people.
You ever use the Medigun ingame? It's great!
Bonesaw is a stab machine.
He can just sit around trying on hats while he heals.
Finally we reach, the shpee.
He can go invisible and stab people, in the real world, assuming he could cloak, why not just stab while cloaked?
Just shoot the guys with the Revolver.
Sapper can turn off a tv.
Stab.
And put on a crude mask to pretend to be the other team.
Probably the most survivable guy.
Ok I'm done.