Which outfit should I use for vault entrance guards?

According to this post and assuming your guard dwellers are already level 50, only two S.P.E.C.I.A.L. stats should be relevant for vault entrance guards.

Perception

Hit chance in combat

Agility

Attack speed.

These are the only relevant stats which have an influence on your fighting skill inside the vault, and there is nothing else the dwellers are useful for at entrance.

The only purpose of your dwellers here is to clear all threats (raiders and deathclaws) as quickly as possible, so I would give them jumpsuits for agility or heavy raider armor for agility and perception and also the best weapons you have (except the ones your dwellers in the wasteland use). If you have no good agility suits or want to to prefer Perception, the (armored/sturded/heavy) vault suits should be your first choice.

If they are not level 50, you could think about also giving them some suits with endurance, so they increase their maximum life and don't die during each deathclaw attack.

You can only assign two dwellers to the entrance room, so they are the only "guards" inside the vault, but it is a common tactic to also put max level dwellers with good weapons in the room(s) right behind the entrance, so they eliminate all threats your entrance guards could not kill.