EBeast, I've been having a lot of mixed opinions of whether Regirock should be taking a A or S slot. I have finally come to the conclusion of agreeing that Regirock should be placed into the S ranking. There is absolutely nothing that competes with its job in-terms of reliabilities as a role of setting up Stealth Rock, becoming a great answer to most prominent Physical (and even certain Special Attackers such as Charizard, Zebstrika, or Regice thanks to its great round of Special Defense) attackers in the tier thanks to its very centralizing defensive typing, huge defense stats that allows it to take on and beat even Pokemon that would carry a coverage move to hit it super effectively, such as Cincinno, Braviary, Zangoose, Emboar, or even Absol, which are roles that any other defensive threat wished they had, and all in all, it bring its potential to an even higher level by becoming a phenomenal team player.
If its defensive capabilities weren't even enough, well, offensively, it is no slouch either. Regirock's really nice base 100 Attack can be utilized to exceptional use. With access to moves like Stone Edge, Earthquake, Drain Punch, Ice Punch, there is not even a single Pokemon in the tier that it can't potentially punish severely to who dare takes it lightly when it comes to attempting to check it/setup on it. To add to this even more, it is an excellent user of Thunder Wave, allowing it to paralyze a large amount of threats that would otherwise normally be able to take on Regirock pretty well, such as Ludicolo, Samurott, and the like. This allows its usefulness for the team to an even bigger impact, as offense teams can be dealt with so much easier if even a single key threat happens to be paralyzed.
All in honestly, there is no reason to not decide upon using Regirock, as it can well function on just about any team, and it has something in common in any of those teams within the fact that it will never disappoint you and will always exceed expectations. So yeah, I'll all for it into the S Rank.