S: Action Hero, Nightcap, Super Brainz.
Nightcap is just broken, Action Hero doesn't have a direct counter, and S.Brainz counters all of the plant meta. A notable mention is Chomper, but Chomper has been reworked so much that nobody can play him like the S he can be because they are treating him like his old GW1/GW2 self.
A: Chomper, Scientist, Engineer, Peashooter.
Scientist has a potent shotgun and powerful heal abilities. Engineer has a strong turret and damaging primary, peashooter can deal massive burst damage. A notable mention is Deadbeard. A player good at maneuvering with his anchor and barrel can play him like an A.
B: Deadbeard, Acorn, Snapdragon, E.Slide.
The oak has massive power, but is a slow and big target. Snapdragon does good damage but he has ineffective range. E.Slide has good abilities, but lacks power in her primary weapon. B and C are pretty close in power. I considered all of them.
C: Space Cadet, Sunflower, All-Star, Cactus.
The space station is powerful if you stack, but the Cadet's reload makes her useless otherwise. Sunflower is an excellent healer, but she has no firepower. All-Star has excellent shielding, but also has low firepower. Cactus is powerful, but only if nobody aims for you. I considered Rose for her extraordinary utility, but she has somehow less firepower than Sunflower.
D: Citron, Rose, Foot Soldier.
Citron is another character with poor firepower. The only reason Citron made it out of F is because of his large shield. Foot Soldier can hold his own in a fight, but stands no chance if his enemy has allies.
F: Imp, Corn.
Imp can't do damage. His primary fire sprays everywhere and never hits. His impkata does close to no damage. Gravity Grenade is tiny, and I won't even start on his mech, for anything it ever did was when it was in the exact right place in the exact right time. Corn is trash. His cob busters have all of the imp blaster's problems. Husk hop does nothing. Tactical butter does only recon, no other utility, not even damage. And shuck shot is impossible to hit, and only does 55 if you somehow land it.
Those are my opinions. I can see shifts around B-D, but I have strong beliefs that the S-A and D-F characters all belong in their ranges.