Just my opinions collected from about 20 hours of BFO.
S TIER - JUNKER QUEEN
Her ultimate does way too much damage. The lightning strikes melt through even Rein and Roadhog like nothing with damage that outclasses every other character by a mile. I see her winning most BFO games and her ult flat out wins in a head to head against any others.
A TIER - PHARAH, ROADHOG
Pharah just does great damage at great range, and flying makes her a pain to duel for anyone besides widow. I wouldn’t call her broken, just really good.
Roadhog has always been S Tier in deathmatch, but falls to A in BFO because his oneshot doesn’t work on half the cast. However, he’s still incredibly tanky, does great damage, and has a very strong ultimate.
A- TIER - LUCIO
Lucio is arguably A tier, but has weaker matchups against Pharah and Hog. However, his mobility, damage, and stun on soundwave let him reliably duel any of the characters below him.
B TIER - REINHARDT
Rein suffers harshly from his range in this mode, with charge being a subpar mobility option on a cooldown that feels much longer in a deathmatch environment. Rein has to run around chasing kills with mediocre tools to do so. His ultimate can turn him into a killstreak menace, saving him from a lower tier.
C TIER - WIDOWMAKER
Widow’s gameplay is mostly the usual click on heads, and that falls a little short when most other characters in BFO have either high mobility, shields, or too much health to one tap. I’ve found her most effective strategy to be farming ult, and while her ult turns her into an absolute menace, it’s about the only time she’s threatening on the field.
D TIER - RAMATTRA
Ramattra feels very weak compared to the rest of the cast. It seems he doesn’t have the 20% movement speed buff in nemesis form, and you can feel it. He spends the match chasing kills without the damage to duel the other heroes. He’s weaker, slower, and more fragile than the rest of the cast. His ultimate doesn’t do enough damage to threaten any of the other tanks, and his game plan is essentially farm ult to kill people with whirlpool. He’d be fine if he had his extra armor and move speed in nemesis form.