imo it’s his action skill
not only does it obviously boost his damage output with 2 guns AND increased accuracy and fire rate
it heals him instantly AND gives him damage reduction, he becomes very hard to kill from as early as Level 5
he doesn’t even really need to invest into tank/defence skills because of this
Zero’s Deception starts out being utter trash in comparison. In-fact, Zero (and other characters) need to actually invest skill points into other skills to do what Sal’s action skill does for free
Sal (and Gaige too) = Borderlands on easy mode (note I’m not talking about UVHM which I don’t play much)