While I don't agree with Keldeo being S-rank, there is no way it is dropping to A. It is easily one of the top A+ 'mons in the meta. For example, you SAY more 'mons wall Keldeo, but really, there are no other common. Also, 5 'mons is nowhere near a large portion. Also, none of the pokemon you listed enjoy switching in on a Scald burn, since it shuts 3 of them down and cripples M-Venu. Only Amoonguss can afford to be burned in a match. On top of that, each one of the counters you listed can be hit with a coverage move. The two grass types don't like HP Flying (especially Amoonguss which is forced to invest heavily in Physical Defense for Azumarill), CBBNite doesn't like Icy Wind, and the two water types don't like HP Electric, which has been rising in popularity. I would also love to see any of these handle SubEndeavor Keldeo. Yes it is prediction reliant, but that is a two way street, and one can use Keldeo to play mind games as well. Also, as far as I can tell, the only 'mon that can take less than 40% from any one of Keldeo's moves is AV Azumarill... Keldeo also doesn't need any more coverage than what it has got, so saying it has a shitty movepool doesn't mean anything. 108 is a fine speed tier, beating every wall in the meta, which it is all it needs to beat since its, ya know, a WALLbreaker. It also outspeeds a huge portion of offensive 'mons in the meta, including the Zards, Landorus, Garchomp, non-Sand Rush Exca, Bisharp, Kyu-B, and M-Garde. As for priority, only Gale Wings Brave Bird and Aerilate Quick Attack are doing a lot of damage, while being able to easily handle Shadow Sneaks, Aqua Jets, Mach Punches, and Ice Shards. So while Keldeo may not deserve S-Rank, it CERTAINLY doesn't deserve anything below A+.
No there are more than those 5 mons (M-Venusaur, Amoonguss, Gyarados, Dragonite, Azumarill) can take hits from Keldeo. Also the lati twins, Sylveon/Clefable/Togekiss, Slowbro, Starmie, Chesnaught, and Vest Tornadus-T and Tangrowth are a few more off the top of my head (notice all of these also OHKO Keldeo back, so no tanking your way through). No, not all of them can avoid 2HKOs in every circumstance but that's where being so specs-reliant sucks. Even if Keldeo manages to nail Venusaur with HP flying, it's not crippled, it can still come in later against something slower and nab a Synthesis. And then the Venusaur user has a super easy time switching to just about anything else out there now that Keldeo is locked into hidden power. It sucks that Keldeo mainly gets 2HKOs rather than OHKOs because opponents can save their half health pokemon and switch in something that resists whatever move it's using.
In particular every regenerator mon in the meta is such a good switch in to specs/scarf Keldeo, because all they need to do is scout the move it's using and switch to a resist, barely taking any damage after they switch out, with their passive recovery they get around Keldeo's way of punishing it with Scald. Well, that and they all have nice typing to take its moves and hit back hard. None of them are too crippled by sub+endeavor either unless it's paired with a pursuit user, because 2-3 switch outs and they're healthy again. Starmie, Lati@s, and Sylveon all can take on sub+endeavor no problem as well.
There also isn't any offensive mon as high ranked as Keldeo with as barren of a movepool. Every Keldeo has a water STAB (or even two, lol) and Secret Sword, and then it's hidden power, icy wind, calm mind, sub, endeavor, and it runs out there. Those pokemon I listed earlier show how water+fighting isn't amazing coverage. It's decent, but it still leaves a lot of holes, since they don't cover each other's resists.
Hidden power and Icy Wind are such low base power that it's so risky to mispredict, since there's maybe one pokemon on a team you're trying to hit with it. Something like Scarf chomp doesn't have as much of that problem, it can, for example, click stone edge against that charizard x to screw the opponent whether they stay in or go to the predicted togekiss switch. But Hidden power and icy wind are so niche in what they hit, there's no breathing room like that, 90% of the meta will exploit it to set up. Calm Mind is good, and life orb+calm mind imo is way better than specs, since you don't have prediction issues which is, if you can't tell, the thing that I think makes Keldeo a poor wall breaker. But even Life Orb sets take away from Keldeo's ability to counter a few threats like Bisharp and Tyranitar
Also, 108 speed looks a lot slower when nearly everything faster carries a super effective stab, ex. Lati@s, Thundurus, Greninja (extrasensory/HP grass/Grass Knot), Tornadus-T, Talonflame, Mega Manectric, Deoxys-S, Alakazam, Raikou. It's pretty easy to check Keldeo when darn near every pokemon that happens to be able to take a hit from it or outspeed also has a super effective STAB, and that makes its decent bulk only useful for switching into the stuff Keldeo checks, rather than tanking its way through its own checks.
