It’s different for each character, you either want to augment their strengths, or buff up their weaknesses, and it comes down to which one feels more vital to succeed.
For instance, on Kleese, I go with shield strength, damage reduction, cooldown reduction, and health regen attributes, because he’s just a little too squishy, cooldown helps keep him alive, and a little health regen lets him stay in lane almost indefinitely. Buffing his bad points so he can stay alive and keep doing his job.
With Montana I go with all health regen items, and health increase items. He’s never going to have a shield for long, and when he gets under %50 his regen goes through the roof. It helps him stay alive and keep pulling enemy team bullets, which is what he needs to do, dealing damage is not my biggest concern. buffing what he does well, because it helps him keep filling his role.
With El Dragon I go with sprint speed, damage reduction, movement speed and attack speed, while he’s good at these, he’s better with more, and dealing damage keeps him alive better for me than him being tanky, buffing what he’s good at.
With Whiskey Tango, I go all attack speed… seriously, I have 4 attack speed attributes, it’s what he really fails in, so that’s what I buff, and then a little crit and a little skill damage.
The point is, it depends on what you need to do, pick what’s going to help you do that better.