Groudon has a bit of problem that I can see why it can't be S.
That's the problem. Groudon's power cannot even be compared to Kyogre. That's why I said Groudon's rank is appropriate. But I won't be against him being S-Rank. He supports better than other pokemons, with Drought being beneficial for the whole team. He has two set-up moves that destroy various teams, who think that Groudon is running a defensive set.
Besides, I was debating Groudon vs Dialga, not vs Kyogre.
Some people want to promote Dialga to S-Rank, but imo, Dialga shouldn't be higher than Groudon, with the arguments I expressed on my last post.
Dialga sure has versatility, but can he wall a lot of threats in Uber? His support set is meant to absorb resisted hits (or weaker hits), such as Kyogre's choice scarf Ice Beam. On the other hand, support Groudon can switch into any Terrakion, Ekiller or various non choice band Outrages, and OHKO back or phaze while surviving the 1vs1. Dialga cannot take many neutral hits. Other dragons have a decent shot at 2HKOing Dialga with their STAB moves. Both don't have recovery besides leftovers.
Groudon can sweep late game with Rock Polish or/and Sword Dance, but Dialga's only set that can clean up late game is the Scarf Set (seriously, pretty much any scarfers can do that), and by using BulkUp+ Resttalk. Tell me who managed to use this set succesfully in this metagame.
Dialga can punch holes with Specs/Life Orb, but any team with the pink blobs, Lugia, tank Ho-Oh (not in rain), SpDef Kyogre (if you avoid Thunder), Heatran, Arceus-Rock in sand, Jirachi in rain can wall it. Outrage helps against some of those pokemon, though. Have you tried Banded Groudon? Sure, you need to predict well, instead of spamming Draco Meteors, but actually the only surefire counters are Landorus-T, Cresselia, and Hippowdon (2,34% chance of 2HKO), physically defensive Giratina, and lol 252/252+ Arceus.
In addition, if Dialga was promoted, when unexperienced players look at this thread, they might think that they must absolutely find a check to Dialga (which they should do), but countering Groudon is even more important. Basically, Groudon has an easier time getting past his counters than Dialga.
Dialga is a powerful pokemon, but does NOT shape the metagame. It's Dialga who adapted to the metagame, but it's Groudon and Kyogre who are defining it.
Edit: C Rank is too low for Froslass. Froslass is a spiker who can beat Deoxys-S. If you lead with Deoxys against a Froslaass, either you break Froslass' Focus Sash and die afterwoods (Icy Wind+Shadow Ball is enough, and you're outsped after a speed drop), either you set up a layer, and begin the match 5vs6 against a sashed Froslass. Espeon and Xatu cannot come on this either, and only Froslass can set up spikes against Forretress. B- is more appropriate, imo.