I would recommend upgrading Red Death and Fang of Ir Yût first. But the order is up to personal preference. -The Red Death is useful in many situations, and it's namesake perk helps even more to stay in a fight. -Vex Mythoclast is very powerful (and in my opinion, a real beauty). But you have to aim for critical hits for this weapon to work really well. -Thorn is very powerful too, and a beast in the Crucible. Possible Two-Crit-Shots with it's special perk. -MIDA is an all-rounder Scout Rifle, and really fun in a speedy Bladedancer Build. But it excels nowhere. -Universal Remote is also fun, and with the recent Shotgun-buff in PvE really powerful there. But a primary shotgun limits you to either very close/very far range (with a Sniper), or very close/medium (with a fusion rifle). So you have some weaknesses outside of the ideal ranges. -Fang of Ir Yûr is a really powerful Scout Rifle, and my preferred day-to-day weapon. High Crit-damage, useful perks, and the best, it free's you an exotic slot due to being a legendary. -At last, Abyss Defiant. An useful Auto Rifle. Powerful with it's slower RoF/higher Impact perk. An allrounder to use, and also free's you up an exotic slot. Hope this helps!