I think rotom-h should be moved up to B rank. It's stab is nearly unresisted and with HP grass he hits even more of UU for super effective damage. Combine that with supportive moves like will-o-wisp and you have a deadly Pokemon. He's still weak to stealth rock and special walls like porygon-2, umbreon, and snorlax wall him, but he can still burn huge holes into teams.
Currently, Crobat is both A and S ranked. I'm sure it's a typo, but with that in mind, I think Crobat should be A ranked. While it is a great Pokemon and extremely versatile, I think it's weakness to stealth rock and lack of usable attacking moves makes him A.
Cofagrigus, however, should be S ranked. He walls a good number of Pokemon like heracross and can set-up on even more. Trick room/NP is extremely deadly. It alone can sweep teams lacking toxic or a healthy special wall. His supportive set is great too. Crippling physical attackers and walls with will-o-wisp and healing off damage with pain split. An S rank poke for sure.
Some other Pokemon that should shift tiers:
Dynamic punch hits like a truck and the confusion hax can lead to a huge disadvantage.
CB and life orb sets can wreck teams that lack a good check to weavile. Pursuit kills all the ghosts in the tiers and ice shard is a great overall stab move. Low kick, night slash, and ice punch are all great moves to add to weavile's arsenal. If unprepared and with hazards down, weavile can take down whole teams.
Outside of spikes, this thing isn't very good. It's extremely frail and weak to stealth rock. While it can get two layers down, it's usage for the rest of the match is pretty much useless outside of death fodder.
Finally, moving colbalion and empoleon to A might be a good idea. Both of them hit like a truck and can take hits in return.