1) Ludicolo down to A.
Ludicolo has fallen in usefulness. The metagame now strongly favors fast, powerful hitters, and Ludicolo simply doesn't have as many opportunities to set up. Simple little changes like Custap Berry's release prevents Ludicolo from being able to use Golem as set-up bait, and many smaller changes like Musharna often carrying Thunder Wave makes life more difficult for Ludicolo. Kangaskhan's massive rise in usefulness also contributes to Ludicolo's downfall, since it can't really sweep without being punished by Fake Out + Sucker Punch (the former which stalls out its rain too, making it that much harder to actually sweep). People have even started running bulky Samurott sets to help deal with the faster threats, and I'm pretty sure FLCL has used a spread that guarantees that Samu lives a Modest LO Giga Drain and OHKOes in return with Megahorn.
In summary, Ludicolo just doesn't have the speed or bulk necessary to keep up with this Normal-heavy metagame right now IMO. Of course, it's still very good, but it's not good enough to merit S-rank right now.
I dunno about you, packing what is essentially the special variant of Gear Shift (albeit only boosting Water attacks for 5 turns) still makes Ludicolo a relevant threat that teams should prepare for to avoid getting bowled over. That said, it is getting increasingly difficult for Ludicolo to safely get that Rain Dance up, nor can it beat dedicated special walls like Regice and Lickilicky, but Ludicolo is still something to consider when teambuilding, as those Rain boosted Surfs/Hydro Pumps with Grass/Ice coverage can be incredibly difficult to stall out the rain against.
2) Kangaskhan to S.
Kangaskhan is fucking phenomenal right now. I used it in like the last five SPL matches I played, and it was useful or the MVP in every single one. Kangaskhan has literally no drawbacks for being a fast priority platform that still hits impressively hard and has excellent coverage. Kangaskhan manhandles offense barring a very select number of Pokemon (Regirock, Carracosta, etc) and even threatens to do stuff like 2HKO Musharna given a bit of prior damage. Kangaskhan owns or has a very solid chance to beat a majority of Pokemon in S, A, and B ranks right now, and that's just with its standard set. Kanga also has a pretty solid movepool, meaning it can swap out Sucker Punch or something for Toxic, etc.
I don't think you're being honest if you can say that Kangaskhan isn't one of the best Pokemon in the metagame right now. There aren't any downsides whatsoever to slapping Kanga into an offensive team, as long as you have a means for breaking down physical walls.
This on the other hand I'm not so sure. Kangaskhan is not the first thing that comes to mind when I think of "top NU threats" (though that may merely be a sign for Kanga to take advantage of). Kanga has well rounded stats and can run a good variety of sets, but are they really all that? I've used Kangaskhan myself and I've found some notable flaws with each one.
Priority abuse, the main set, aims to wear down faster sweepers and break down slower ones. The problem: it doesn't do quite a stellar job at both. The coveted Fake Out-Sucker Punch combo easily takes a toll on Kanga herself if running Life Orb; if she's running Silk Scarf instead for better survivability, suddenly Kangaskhan loses a notable amount of power, such as missing the OHKO on Bastiodon with Earthquake, or not 2HKOing Musharna (as you so claimed)
after SR. Life Orb isn't priority-friendly, whereas Silk Scarf isn't power-friendly: hell, I even find myself struggling to 2HKO Ludicolo with Fake Out + Sucker Punch, the latter which can be exploited.
Choice Band sets now have the power to break through most bulky pokes and has more freedom in spamming Normal moves thanks to Scrappy, but it sacrifices strong reliable priority as a result of being Choice-locked, taking away one of Kanga's trademark offensive traits.
Substitute sets are interesting, as they allow Kangaskhan to bypass the bulky pokemon it has trouble getting through before. From there, SubPunch or SubToxic sets can work their magic. But oh boy, if you thought the Silk Scarf set can be lacking in power, this set merely scratches the same bulky pokemon it is trying to wall. SubPunch sets will not be able to break through most of the pokemon it can set up its Substitute on, while SubToxic sets can run into coverage issues and pokemon that do not mind Toxic (Heal Bell Misdreavus and Vileplume).
Yes, Kangaskhan is good, but not S-Rank dominant. She can't dish out the same kind of punishment for guessing one of her numerous sets incorrectly like Samurott does due to being purely physical, nor can she break through bulky pokemon like every single other S-Rank poke can due to her inability to boost or hit insanely hard off the bat. I'd probably settle for her in A-Rank, but let's hear more discussion on this.
3) Haunter to S.
This one might be a little bit harder for me to push through, but Haunter has also been consistently one of the best Pokemon in NU as of late. With a great Speed tier, impressive coverage in its STAB moves (and HP Ground to give it perfect coverage), Destiny Bond to ensure at least one kill, and a wide variety of other moves at its disposal, Haunter is one of the best offensive Pokemon in the tier right now. Primarily, Haunter's usefulness comes from having very good coverage and power to threaten slower offensive teams, and Destiny Bond to lure things like Regice or Metang or Skuntank and quickly and easily remove the opponent's counter to Haunter's teammates. Haunter should often be paired with stuff like Kadabra or Gardevoir because of how easily it can lure and beat these. It's also a solid Scarf user, outspeeding most other common Scarfers and having both Trick and Destiny Bond to screw over defensive and offensive teams alike. Other possible sets include: Wisp, Hypnosis, SubSplit, SubDisable (still pretty mediocre right now tbh), Specs...
Haunter is a pretty threatening force alright. Unlike Kanga before it, it can cause headaches for defensive teams due to its numerous disruptive options mentioned above. It is incredibly frail, but has 3 immunities and a couple of handy resistances to Grass (allowing it to halt Sawsbuck if at full health) and Bug that can make up for it, or if worst comes to worst, utilize Destiny Bond to KO an opponent you otherwise couldn't have.
The cons of Haunter is that its fraility leaves it vulnerable to almost every Scarfer or priority that hits it neutrally. It also faces a certain degree of competition with Kadabra, who is faster, stronger, and can be equally disruptive with Encore and Magic Guard. However, Haunter has a easier time bringing down Skuntank than Kadabra, KOing with Destiny Bond to allow for Psychic teammates to sweep unhindered. Mixed between S and A-Rank, though I feel its tremendous fraility leans it towards A.
There are a few other Pokemon sitting in S that I kind myself leaning against but not strongly enough to actually propose their removal right now (Braviary, Zangoose). A tier looks pretty solid, but B has a whole bunch of wackadoos that probably belong mostly in C (with a few exceptions that might even be A).
I've seen people give praise to Seismitoad, but haven't seen him being proposed for a higher ranking. Perhaps he could be a viable A-Rank canditate?