Looking at that list you put together, 5 out of the first 7 Pokemon you listed can OHKO Crobat after rocks if they anticipate the switch and use the appropriate move. That's not exactly what I'd call "being able to switch in with little trouble". Out of your list of walls/tanks, Crobat will have to risk being burned by Swampert, Suicune, Milotic, Blastoise, and Dusclops if it switches in. If Suicune uses Calm Mind as you switch in, you lose whether it's Offensive or Resttalk. If Togekiss uses Thunder Wave or Nasty Plot, you lose. Whether you beat them 1 on 1 is irrelevant because that situation is rarely going to present itself. Even if it does, you're not going to come out on top every time; there are too many variables that could change the outcome like Scald burns, non-standard movesets, etc. I can tell you right now you will flat out lose against Swampert and offensive Suicune. Crobat is a shaky check to pretty much every metagame relevant Pokemon you listed in that post, so I don't see how you can argue that it's S-tier material. I did say that its most effective sets are offensive, which means I don't think its defensive sets are effective. I'm skeptical of any defensive Pokemon that bears a weakness to Stealth Rock. In Zapdos's case, if Stealth Rock is on the field Flygon's Outrage, Heracross's Stone Edge, and Ambipom's Return are all 2HKO's, and Zapdos is slower in each case. If it requires a Spinner to perform decently, then it's not S-tier material in my opinion. I'll give you Sub Roost Zapdos though, I overlooked it and it is one of Zapdos's best sets. Looking at the usage stats for September, 71% of Zapdos were either Timid or Modest, so the only situation in which I'd consider my opponent might be using a defensive variant is if I see they're using a stall team. So really, Zapdos has Heat Wave and Raikou has Calm Mind, and for typically offensive Pokemon Calm Mind is substantially better. Okay, about the Nidoqueen-Nidoking issue. It basically comes down to Nidoqueen's bulk vs Nidoking's Speed, since they are otherwise the same Pokemon. While Nidoqueen's bulk isn't significantly better than Nidoking's, it does help it considerably to check Heracross and Mienshao, which is a big deal in this metagame. Queen can also take an unboosted Extrasensory from LO Raikou and a Hidden Power ice from LO Zapdos while Nidoking cannot. The advantages of Nidoking's additional Speed are basically limited to outrunning Chandelure, because everything else comes down to a speed tie. The ability to reliably check 4 extremely dangerous Pokemon outweighs Nidokings capacity for outpacing Chandelure, in my opinion. Nidoqueen is not drastically better, but it is better, so I think it should remain in the A-tier and Nidoking should remain in the B-tier.