Hmm. Depends on the cave and the type of monster.

A flamethrower has several advantages: it creates light, doesn't require accurate aiming, and it's great for clearing out passages ahead of you as you make your way to the surface. The disadvantages are, of course, it sucks oxygen, weighs a ton, and a single claw in just the right spot on the tank and you go BOOM the end game over.

A shotgun might be good: easy to carry, heavy damage, good in close quarters, little worry about ricochet. But how much good will it do you in total darkness? Or can we assume a lantern or mining helmet to provide illumination?

Flaregun?

Ooh, I know: the shish-ke-bab from Fallout 3: flaming chainsaw! Perfect!

Lol, that was one of the inspirations for the discussion, Hubby was playing Fallout New Vegas and trying to figure out the best weapon to use on the Deathclaws. He concurs, flaming chainsaw is a great way to go. I like the elegance of a flare gun to the face.

In WW1 they used entrenching tool and grenades for trench warfare. Grenades might cause cave-ins though.

I think soft-nosed bullets don't ricochet much, so they might work. I think I read somewhere that ricochets aren't that much of a problem in real life, as ricochets don't tend to bounce straight back, just skitter sideways.

Short sword? Full auto shotgun is powerful but cumbersome. The FN P90 is nice and compact, it's used as a sidearm by tank crews and so on. Chainmail armour could be good protection against biting. Cattleprod or electroshock baton might work well against naked creatures.

An elvish sword of great antiquity.

A gas mask/protective clothing and probably sarin gas or another nerve agent, maybe an asphyxiant or a combination of a few fast-acting deadly gases, depending on the monsters.

A sneaky and immediately deadly poison, hidden in a food even more irresistible to monsters than we are.

Alternately, a cross bow with a perfect aiming mechanism and awesome flaming/frozen/enchanted arrows.

Alternately, a cross bow with a perfect aiming mechanism and awesome flaming/frozen/enchanted arrows

I could wear my +5 Agility Spelunking Helmet of Fentoozler and equip my Bow of Illuminated WooWoo so I get the 2%Crit perk. That way I am less likely to be eaten by a grue.

A dragon. A fire breathing dragon. A HUGE fire breathing dragon.

If you're fighting Deathclaws, the best weapon is a sniper rifle from a high point a mile away. Sez the guy who juuuuust lost about an hour of gameplay by cluelessly wandering into a Deathclaw nest and getting his face ripped off. I had a 10mm submachine gun, a .44 Magnum, and a machete. Might as well have been tickling them with a feather duster.

As always:

When in doubt, Fireball.