Personally, I can't see how Ferrothorn could possibly be A-ranked. I never had trouble with it, and when I used it, I found it consistently underwhelming. It's murdered by pretty much every Fighting-type in the tier, which isn't exactly doing it any favors. Hell, I've been able to exploit its 4x Fire weakness even in Rain. As for tanking Dragon moves... The only Dragons I can think of that don't carry Fire moves often are Kyurem-B and Rain Dragonite. As a Spike-stacker, I found Deoxys-D to be 100% better 100% of the time since Deoxys-D has actually usable Speed and Taunt. Pokemon that beat Ferrothorn? Well, Taunt from Deoxys-D shuts it down completely, but there's also Breloom, Lucario, Volcarona, Heatran, Genesect, Keldeo, Terrakion, Magnezone, Ninetales, and.... Okay, the list is already massive. Basically, OU is full of crap that Ferrothorn isn't so good against. I mean, yeah, it has a load of resistances (naturally, it's a Steel-type), but its two weaknesses run rampant in the tier.
In my opinion, keep Ferrothorn in B rank.