Based off performance and some theorycrafting. A lot of the lower tier heroes are uncommon to see, so I had to make some estimates based around their kits and how well they stack up against different kinds of lobbies.
EDIT: it’s not in order within a tier
I want some of whatever you’re having. Reaper in S tier above both Cass, Tracer, and Hog.
BOOYAH, WE’RE NOT LAST, WE’RE NOT LAST 
(Deathmatch Weaver here, also Brig who was my previous DM OTP)
I would give my input, but I don’t think I’m qualified. Especially since I’ve actually quit the mode thanks to all the stories of report abuse and false bans happening there ( 
), so I have no current experience
why in God’s name is Reaper in S tier tho
Just added an edit that it’s not in order in each tier. But ye Reaper still somewhere in S, mainly because of three honestly ridiculous people that main him and are nigh unbeatable. I rarely see Reaper outside of them though, so it was hard to judge.
If I had to order them then it’d probably be Hog #1
I have no idea about the current Deathmatch meta; however, back in the day, he was definitely up there just to shred Deathmatch Hog abusers.
I actually ran into so many full Reaper lobbies that I joined them. But then they went Cassidy. 
It still sort of boils down to that sometimes. Roadhog is in S cause he kills mostly everyone with ease, so then Echo/Reaper/Tracer end up being the best to at least have some chance of fighting him/running away, which then results in Cowboys showing up to kill those three. Then Hog to kill cowboy and so on.