While I guess I'm not surprised to see him so low, I think Registeel deserves a C rank. It shuts down all Raikou between Thunder Wave and Earthquake (providing they arent Volt Switching) Can switch into offensive Zapdos, paralyze it, and get SR up before it kills you, or break defensive ones subs and play the prediction game to get status on. Really only fears repeated Hydro Pumps from Kingdra, as you can break its subs and paralyze setup versions. While youre unlikely to beat Togekiss, Roserade or Shaymin 1-on-1, he can easily paralyze and force them out (yes I know they have natural cure but it still steals the current momentum away.) Also a near perfect switch in to Yanmega, which is pretty rare
Walling one-dimensional attackers isn't amazing, but he does it well and the ones he cant beat by himself are often severely crippled by TWave; indeed this is often his most threatening quality. Maybe I just have a lingering fondness for him from DPP UU, but I never regret using him on my teams. His ability to set up rocks, spread status and counter/check so many special threats all at once can make for an invaluable support mon. One worth of a higher ranking than D imo. Interested in hearing arguments against this!