Duelist
SSS+ - Neon (abuse while you still can)
S - Yoru , Iso
A - Jett
B - Raze
C - Reyna (just play iso)
D - Phoenix
Controller
S - Clove
A - Omen, Astra
B - Harbor
C - Brim
D - Viper (no icebox or breeze, really just no reason to pick her on any map over the other options)
Sentinel
S - Cypher
A - Deadlock (surprisingly good if you know what you're doing), Chamber
C - KJ, Sage
D - Vyse (only playable on abyss really, even then cypher better)
Initiator
This class is very very map dependent, take these rankings with a grain of salt
A - Sova
B - Breach, Fade, Skye, Gekko,
C - Kayo (Insane in an organized setting, very hard to get good value out of in ranked)