Most of you guys seem to be on board with Keldeo and Lando-T being S rank, so I guess I'd better make the case for Latios. I was actually very sceptical about Latios being S when it was first brought up, however, if we're using standards that would allow Torn-T, Keldeo and Lando-T to be S rank (and we probably should), I think Latios should be S too. I think it's about on par with these three, both as a metagame-defining threat every team needs to account for, and as a really great pokemon to use that will usually pull its weight in battle.
First of all, it's about as splashable as you can get, not just becuase of Defog, but also because it combines a ton of useful resistances with great offensive prowess, it can weaken or lure a bunch of Pokemon, it can seriously damage teams without needing much support, and most of its checks are straightforward to deal with and have pretty viable checks of their own, so you rarely have to accomodate for it by changing your team round when you want to use it. It's one of the most common Pokemon in OU for a very good reason, and when you compare it to Lando-T or Keldeo it actually suffers less competition for its role, or rather, less viable competition : Keldeo has to compete with Azumarill and Manaphy for the role of offensive Water while Lando-T is very simplar to garchomp in what it does, and all 3 of these are pretty dominant threats. Meanwhile, Latios has Latias and Starmie, both of which are generally worse options. My point is, if you want to use sompething like Latios, you're almost always going to use Latios, and that's something that's less true of Lando-T and Keldeo.
The biggest problem with Latios, and the most convincing argument against it moving to S rank, is the prevalence of Pursuit. And yes, the fact that Latios can just be useless vs certain teams defenitely sucks, especially if we're looking for consistency. However, it's not like Pursuit has no counterplay at all, you can use the opportunity to get a free setup or hazards up. Pursuit users are either easily taken advantage of (TTar, Metagross) or have a lot of trouble actually switching into and trapping Latios (Weavile, Bisharp), so there's always a way around it. Also, Pursuit trappers can't actually prevent Latios from Defogging, so it's not completely useless and can still help you win a battle even if it gets trapped.
In fact, Defog almost guarantees that Latios is going to at least be able to do something in a battle. And if you ignore teams with Pursuit trappers, Latios can put in work pretty consistently in general. Like Keldeo, many teams have multiple checks to it, but like Keldeo, it's still able to put in work against teams that are prepared for it. And unlike Keldeo, and even Lando-T and Toprn-T to a lesser extent, it can threaten multiple types of teams at once.
Keldeo can threaten offensive teams with a Scarf and bulkier teams with Specs or SubCM, but it kinda has to choose : Specs and SubCM aren't very good against most offensive teams due to Keldeo's underwhelming speed tier, while Scarf just falls flat on its face vs any bulky team (aka any team with a Clefable on it).
Lando-T sorta has the same problem where Dual Dance and offensive SR setter sets struggle vs offense due to their limited defensive capabilities, but defensive and scarf sets are too easy to switch into and wear down to really be hard for bulky teams to handle.
Heck, even Torn-T has to choose between screwing over bulky teams with LO+Taunt or actually switching into offensive threats with AV. But Latios only really has to choose one set, becuase it only needs one set, one that can switch in on offensive threats and put offensive teams in a tricky situation due to the pretty low number of Dragon resist that actually work on offense, and which can come in multiple times against bulkier teams and chip away at its checks until they can no longer handle it.
Latios doesn't really have to make compromises bewteen threatening team type A and team type B becuase it can do both at once; yes, it has to choose between HP Fire, Roost and EQ sometimes, but none of these are really necessary in order to be effective against a certain team, they just make it easier. And even if Latios faces a team with 3-4 checks to it, it can always fall back on Defog, much like Lando-T can fall back on SR, Keldeo on burning stuff, or Torn-T on its pivoting capabilities.
Latios is something you'll rarely regret using, and that's what makes it S rank worthy more than anything.
Now, a while back, I said I wasn't sure about what the standards for S rank from now on. But now, after thianking about Latios for a while and comparing it to other S rank nominees, as well as A+ ranks in general, I have a pretty clear idea of it, and this final point I want to make about Latios will lead into that, so buckle up.
Okay, so the main reason Latios is so splashable is becuase, alongside its offensive power, it's also capable of supporting its team and switching into things. Granted it's not great defensively, it has trouble switching in on Keldeo more than once for example. But you can say the same thing about Keldeo and Bisharp or Weavile, after all, Knock Off + +2 Sucker Punch and Knock Off+Icicle Crash+Icicle Crash can all KO Keldeo. Same goes for offensive Lando-T variants (particularly Scarf) and LO Torn-T. But the fact that something as threatening offensively as these 4 can switch into such powerful threats is pretty remarkable in the first place.
And to me, that's what the difference bewteen A+ rank and S rank should be. In A+, you have the best offensive threats like Weavile and MLop and MZam, and the best defensive and supportive threats like Heatran and Ferrothorn. What puts a Pokemon above that and in S rank territory is not just excelling offensively, but also being capable defensively / supportively within the same moveset, or vice-versa (and of course, this has to be relative to the metagame, and whether or not said offensive/defensive capabilities are actually effective/useful in it). Torn-T and Lando-T's offensive sets, Keldeo, and yes, Latios, all fit these criteria, what with being very powerful offensive threats with greater defensive capabilities than your Bisharps and your Excadrills.
This also means that Torn-T should be S more for its LO set than anything, and that Lando-T's defensive set (great defensively+deals pretty solid damage) and dual dance set (legitimately terrifying offensively+can switch into a few things) are the sets that make it S rank. Which makes sense to me at least, since those are definitely the sets I've had most success with.
So, if we look at the current S ranks with these criteria, things get interesting. Clefable is clearly excellent as a defensive and supportive threat, and can put a lot of offensive pressure on teams with Calm Mind, so no complaints there.
Sableye isn't really capable of much offensively though, it's just a pure support Pokemon, I guess you can run Calm Mind but there's a reason it's not as popular, and that's becuase it's not that threatening, so a drop makes sense from that perspective.
And then we have XZard, which is undoubtedly a huge offensive threat, but it doesn't stand head and shoulders above A+ or anything, and the DD set really isn't very good defensively outside of cheklcing Electrics which just Volt Switch out anyway. While it can run a defensive set, its weakness to SR makes it a less than stellar defensive Pokemon in general, and nobody in their right mind would argue that the set that makes XZard S rank is anything other than its DD set. So yeah, XZard should drop too basically.
And finally, Azumarill should be S rank. It's excellent offensively and well above-average defensively for an offensive Pokemon, so it easily fits my criteria for S rank. It absolutely thrives in an offensive metagame like this, not just becuase of how hard it is for offense to handle, but also becuase of how easily it fits on offensive teams, due to the fact that it checks a bunch of huge threats to offense. It's as good as if not better than Keldeo imo, mostly becuase of how important priority is for offense nowadays.