You should send vault dwellers with the highest S.P.E.C.I.A.L. Stats into the wasteland. However, if you have just started a new vault in Fallout Shelter, I recommend sending dwellers with high perception and Agility into the Wasteland. This is because the Perception stat affects how many key points of interest the dweller will discover while in the wasteland. Moreover, Agility affects the dweller's rate of fire, with firearms increasing their survivability. Dwellers with high Endurance are also a good option as they have high health. However, sending your strongest vault dwellers on quests and into the wasteland will decrease the strength of your vault's defenses. You'll likely equip these dwellers with the best gear. However, if you are playing in Survival Mode and your vault is still in the early stages, I suggest that you ensure that you have spare gear in your vault to be prepared in case of a vault attack.