Is an "S" viable in all content? Viable in steel path and beyond? Viable in the circuit/archons/arbitrations/netracells? Viable in endurance?
Are you including what they are capable of with just the base kit plus augments? Or are helminth and archon shards and arcanes factored into it? Are the upcoming augments, which make Chroma and Loki better a factor? Is the Inaros rework?
Caliban for example really needs some attention and is likely in the C/D range, along with Atlas. They're probably the two worst when announced changes are factored in, but as a Valkyr main it saddens to say she is probably the next in need of attention.