In my opinion;
S - Voridus
A - Cutter, Kinsano, Serina,
B - Shipmaster, Decimus, Forge, Atriox, Jerome
C - Pavium, Colony, Anders, Arbiter,
D - Johnson, Isabel, YapYap
Notes:
-currently, UNSC seems to posses an advantage at most stages of the game. Additionally, Marines/Flamers remains one of the strongest openers
-I would have liked to put YapYap higher, but I don’t find he has a reliable counter to Marine/Flamer spam. With how frequently this opener is seen, I had to place him so low
-all the level C leaders seem pretty balanced to me. There’s some minor issues, but nothing gamebreaking. Also, I have seen some excellent Anders/Arby players that really use them to their full potential, but on average, those leaders don’t seem that strong compared to the others I listed
-Shipmaster is still one of the strongest leaders in 1’s, but his early game isn’t the greatest, and it takes him some time to build up. Still, Shipy comfy on 2-3 bases, against most leaders, will usually win. Additionally, Shipy is a difficult leader to play and very few people can play him well
-the rest of the level B leaders also feel pretty balanced; some performing better in different setting or areas than others. Decimus has excellent early-mid game, but falls off as the match goes on. Forge is still solid; strong hero unit, good leader wheel, and a strong drop. Jerome has one of the strongest hero units and a pretty solid leader wheel. All good choices
-the class A leaders all have similaritie that make them strong; good drops and good passives. Cutter/Kinsano have the best Marine/Flamer opening, and they both provide excellent mid-late game with their leader wheels. Serina also has a strong opener with cryo troopers (with a nice passive buff), solid leader wheel, a good drop, and Kodiaks/Wolves are incredibly strong with her.
As always, this is just my opinion. I’m not a top tier player, or anything if the sort, take this as you will.
Thanks for reading :).