Not to start another new conversation but we can't just nominate every single bulky water type out there to D-Rank because "rain makes it better"
If we do nominate a bulky water, I'd say Milotic over anything else because it's RestTalk set is honestly the only RestTalk set that can be run anymore because it gets a +50% Def boost while statused so it isn't a sitting duck without a Chesto Berry and can run Lefties instead
Milotic outclasses Suicune in every way except for Calm Mind and Sleep Talking a Calm Mind can actually screw you over, not help
Also saying "x has a OU analysis so it's viable" isn't an argument, a lot of OU analyses for non-OU mons are leftover from BW1 OU (Medicham for example)
I honestly can't say I support Blastoise, Suicune, or Swampert moving up, they may have the smallest niches ever but I don't think it's enough to warrant a D-Rank
Never used Flygon or Nidoking, no opinion but those niches actually seem relevant over others
Walrein is C-Rank for Hail Stall, I'll take that
Ludicolo is another one of those OU analysis =/= OU viable, you pretty much said it yourself, it's from BW1, the analysis says his only niche is manual rain sweeping and that's a god awful idea that will almost always have you about 2 turns behind the opponent, I don't know any good reason to use him in OU so I don't agree there either
I understand why we can't put every rain boosted bulky water in the viability ranking thread, hence us not putting something random like an Hydration Alomomola, or something random along those lines.
Let's take a look at Miltoic vs Blastoise and see its pros/cons that makes it viable/inviable
+RestTalk set can run Lefties well and when statused is a pain to kill, and it can take a couple hits from some pretty powerful Pokemon when statused/rested.
+Can pHaze -- unlike opposing water Tentacruel who can stop random setup from some stuff like SD Garchomp who think they can setup on you.
+Has the "Defensive" role better than Blastoise thanks to its bulk and Marvel Scale, without having the weaknesses of Tentacruel.
+Lack of Ground/Psy weaknesses of Tentacruel
-Doesn't really offer much utility outside of pHazing and being annoying
-Jellicent can do 60% of what it does +Spinblock and burn.
-Setup bait to Sub+setup sweepers like SubSD Garchomp and SubDD Gyarados since Milotic doesn't have Roar (it has Dtail)
(this is probably the most important thing, lets it fight from the overshadowing of Jellicent)
+Can pHaze Sub+setup sweepers seperating it from Tentacruel (the main spinner)
+Rain Dish+Leftovers gives it some nice recovery to not rely on RestTalk
+Bulky, not as bulky as Milotic but still is pretty bulky
+Lack of Ground/Psy weaknesses of Tentacruel
-Loses out in natural bulk vs Milotic
-Recovery is reliant on Rain Dish+Lefties
-Outside of pHazing, somewhat a sitting duck without anything to spin.
-Jellicent can do 50% of what it does + Spinblock and burn)
Also one thing I gotta say going for both of them against Jellicent is the lack of susceptibility to Pursuit. Blastoise' main thing going for him is Rapid Spin, which only really helps if you have a problem with hazards + Toxicroak in rain. The niche is really really small, but it's still there and its practicalities are only what I previously said, but something like Milotic doesn't really offer much utility outside of natural bulk, and is even harder to fit in teams in comparison to Blastoise(If i didn't want spin, i'd run Jelli).
In the case of Swampert, it isn't meant to be a bulky water; rather, it is meant to be a bulky choice band wallbreaker which opens holes in a rain team for a mon like Tornadus-I to sweep. A lot of Pokemon i'm mentioning are rather niche, but the niche is existent and should at least be put in D-Rank (once again, Togekiss is there cmon)
I guess I was kinda sounding a bit off when saying "X has an analysis so its viable", and I understand those are the reasons for Medicham being unviable (as you said). But if you check the updates on C&C for the Pokemon i mentioned (Ludicolo/Suicune), you can actually see that these are BW2 analysis; and somewhat recent ones at that. Suicune has more competition with Keldeo then it does with Milotic, and that CroCune is still pretty good. It's viable and has been used on a couple of my ladder teams (ik ladder isn't the greatest achievements, but it was successful there meaning it was at least somewhat successful) Unless PEMN applies to Pokemon to a large degree, I don't understand how Suicune can not be put as viable. Ludicolo on the other hand is something i really don't know much about so anyone else can take the wheel there.
On a side note: a SmashPassed Nidoking is the scariest thing you will see in your whole damn life.
EDIT: Yeah i'll add that lack of recovery part. but then again, Rain dish+Lefties and tenta uses the same thing, but comparing to Jellicent yeah i get it