Note: I have not played all of the characters, though I do have them all unlocked. From an Incursion only standpoint the characters that actively give me trouble are:
Going further basically if I go into a match and my team has one of Miko or Ambra while the other team doesn’t have either I have found the game is simply over. I think supports are incredibly relevant, basically without the main healers you will auto lose and IMO the best team comps have 2 supports (this is where I see Reyna shining over Miko + Ambra). The value of being able to poke and clear waves then run back and get healed is just incredibly strong.
Skipping Gal, I think Ghalt is an incredible tank and just so good at defense. Tanks are much more relevant with multiple supports as well. Oscar Mike just seems solid as a DPS while also providing good wave clear. Maybe other characters are better for specific roles, but hes versatile and can provide a solid medium range element as compared to the multiple snipers and melee characters and pairs quite well with a tank or two and 2 supports.
So taking my experiences on Incursion only I would rank champs like so (in tier no particular order) in importance:
S Tier: At least 1 required on a team
A Tier: Extremely good picks with no clearly defined weakness and lower skill floor
B Tier: Solid champs, many can be comparable to A Tier, but quite a few have higher skill floors or distinct disadvantages
C Tier: Clearly defined weakness or just weaker kits
Shayne and Aurox
D Tier: Bullet Sponges, I have never seen these champs do well