I don't think Rotom-F deserves S-rank. Before the introduction of hail it was a decent pokemon, similar to other ice-types - great offensive STABs and decent stats held back by poor defensive typing and a weakness to Stealth Rock. The introduction of hail made it a lot better, allowing it to use a 100% accurate Blizzard over the weak Hidden Power Ice, along with residual damage helping it gain KOs and better utilize Substitute + Pain Split. However, for this to work out, you have to use Snover. Setting aside a teamslot for an arguably useless pokemon is a huge amount of "support", and considering Rotom-F needs Snover to function properly, S-rank just seems too generous to me.
Also, JirachiCelebiMew, Snover isn't like some pokemon in D-rank which only fill a niche role on some teams; Hail is a massive amount of support and single-handedly makes a lot more pokemon viable, as well as others (Musharna) a lot easier to deal with. I'd argue B-rank, I guess.