I've been pretty doubtful of Landorus-T's S rank worthiness for a while now, although it is a great pivot with great utility and is extremely splashable, it's hard for me to think of a good reason for it being ranked above certain A+ ranks, more specifically, Keldeo, Clefable and Heatran, pretty much the 3 best A+ ranks atm imo besides maybe Talonflame and Bisharp. All of these Pokemon provide great utility, check very dangerous threats, are able to outlast their checks, have a good amount of variety in the sets they run which slightly changes their checks, and are obscenely easy to fit on teams due to how much synergy they have with such a large portion of the metagame, but suffer from the fact that they are checked by a bunch of common Pokemon and that metagame is really prepared for them. And this all applies to Landorus-T too. Gliscor, Phys Def Garchomp, Hippowdon, Chensaught, all are becoming more and more common and can easily switch into Landorus-T. And yes, it can wear down its checks with U-Turn, but it's pretty darn easy to take advantage of that, with Pokemon like the aforementioned Rocky Helmet Garchomp, Ferrothorn, Rocky Helmet Alomomola and Slowbro, even stuff that can't switch into its other moves like Talonflame, Heatran, YZard, Volarona etc because Scarf Lando-T really dislikes having to click EQ or even worse, Stone Edge, since this causes a big loss of momentum which can be fatal to the point of being worth sacking your Talonflame to a Stone Edge from Scarf Lando-T if you're using an offensive team. Scarf Landorus-T wishes it could just spam U-Turn mindlessly, except in the current metagame, it can't really do that and that's a problem.
And yes, there's the physically defensive set, but that set also got a bit worse, mainly because of competition from Hippowdon and Garchomp, who have the distinction of being actually able to counter Bisharp which Landorus-T unfortunately cannot handle (this is doubly problematic for Lando-T since these two are great checks to the Scarf set). Yes, Lando-T does have U-Turn but it's so easy for a team to to be weak to Bisharp that running Garchomp over it is often worth it. There are other sets like double dance but those are not common and have to lose something important, usually the ability to pivot which is why Landorus-T is S rank in the first place.
So many teams just have no trouble handling Landorus-T since they have multiple Pokemon that can repeatedly eat up U-Turns and Lando can't chip them away as fast as it dies from repeatedly switching into attacks + SR. It can outlast its checks, but mainly the offensive ones, against defensive stuff like Hippowdon and Clefable it has a lot more trouble and also it isn't breaking Gliscor ever unless it runs HP Ice. As I said, I just don't really see what puts Landorus-T above the 3 other Pokemon I mentioned and on the same level as Metagross (which has an incredibly limited pool of checks), Sableye (which completely invalidates entire Pokemon and strategies) or even Lopunny (which can break past its checks with utility options like Encore or PuP without really losing anything). If Keldeo moves to S rank I can definitely see it stay in S because it's pretty much on par with Keldeo in general but assuming we don't decide to lower the standards of S rank I think should drop. At this point it would probably be a good idea to make Metagross and Sableye the only S ranks as has been suggested because they're clearly just above the rest.
Haven't seen much Celebi discussion so I would like to chime in : This thing is really good and I support it moving to A, it is very easy to fit on teams for multiple reasons : it handles huge threats like Keldeo, Lopunny, Manaphy and Mega-Slowbro, it has excellent synergy with a lot of very common Pokemon (Celebi+Keldeo+Heatran in particular may be the single best FWG core in the metagame since it holds its own against almost all of the tier), and can be tailored to beat what you want or carry whichever of the myriad support options benefit your team more. Nastypass and SDpass are brutal and can turn your revenge killer into an absolute juggernaut, but the LO set is also really solid and is a really effective lure since Heatran and Bisharp almost always feel safe switching into it. It's still quite rare on the ladder but higher-level players are starting to pick up on how good it is, as indicated by its 14% usage in SPL so far, so I imagine it'll get a lot more common on the ladder too since that's usually how these things work. The only thing possibly holding it back from being A is that the metagame is starting to adapt to it a little, I've seen quite a few HP Bug Keldeos (which is a really legitimate option) being used which sucks for Celebi since it's supposed to be a perfect counter to it. But that's not really different from Latios sometimes running HP Fire or Landorus-T sometimes running SD for Ferrothorn, defensive Grass types tend to get lured somewhat easily, but that doesn't detract too much from their great resistances, especially for Celebi who resists Water, Fighting, Ground, Psychic, and Electric, offensive types which range from "common" to "everywhere". A lot of people look at Celebi's weaknesses and are immediately turned off by it, but that's not what you should be looking at, you should be looking at what it resists, what it beats, and how it supports its team, and those are things it excels at.