Don't forget that the S rank is reserved for Pokemon who can sweep or wall significant portions of the metagame with little support, which isn't the case at all with Groudon. He got a lot of check and some hard counter, there is no reason at all to put him in this rank.
PS : if you counter, he can just SD again and laugh at you. Stop it, really.
Also, I would like to propose Fightceus in the A rank. Despite the fact that he doesnt bring anything really usefull on the defensive side (grass can check both Ogre and Groudon, Steel check Dragon and got a powerful mono-stab, Ghost is ... broken and check everything), he got a powerful Stab in the form of Fighting Judgement which can 2HKO without Calm Mind a lot of thing. When you take at the 'Mons who can check him, they are very few. Darkrai (if he didn't used Dark Void, and you check it easily with thing like Sleep Talk Ogre, then he's a setup bait), Giratina (he can only phaze you, he's 2HKOed by Ice Beam if he miss Dragon tail, he's done), Giratina-O (who dont even have a recovery move), Ho-Oh, Lati@s and Mewtwo, Lugia (who's 2hkoed with SR, so he has to chose between phazing or toxic then die), Kyogre can phaze you, but if he isn't Specs, there is a high chance that you beat him; Deo-A and Ghostceus stop you cold too.
A lot of them are easily checked by Stealth Rock, Rain, or the simple combinaison Jirachi + Kyogre.
The fact that he is really difficult to stop him (which follow the description of this rank), has a very good Stab for the Uber metagame and can check the overcommon Dialga/EKceus, I think it deserve to be A rank and not B. Also, the fun fact is that he's the only Mindceus who can stomp Ferrothorn if he dare to show his face.