In my opinion, Tier A should be splitted. I mean Slowpoke, Porygon, Mienfoo, Spritzee, Medi and Swirlix all deserve a mention over the other Tier A. Also, I don't think Pawniard deserves to be Tier S assuming these don't.
Also, I think Foongus deserves Tier B at least, joining the go-to regenerator core with foo and slowpoke. I would also personnally put Ponyta in Tier C, as its set is quite lackluster and predictable, Vulpix is better in a similar role (which is actually being Fire type).