Tangela: You're denying the fact that Ferroseed also switches into Golurk and Stoutland (with a little prediction) alongside other physical threats including Skuntank, Dodrio, and AlolaSlash. If I wanted a Grass-type physical wall on my team, I would be going with Ferroseed over Tangela 90% of the time, given its invaluable support capabilities. This is because Ferroseed can take on a variety of special attackers that Tangela straight up loses to. Tangela is more of a catch-all defensive pivot that performs only one role that Ferroseed can do equally as well. Also, Ferroseed has an immunity to Toxic, a very important defensive asset that Tangela lacks. I don't know how to make this argument without comparing the two because Ferroseed dropping is the only reason I'm making this nomination in the first place.There's a lot I want to talk about in this post because theres a lot I straight-up dont agree with.
I woke up and I read this and I was thinking "the only thing comparable about Ferro and Tangela is the fact that they are small-looking grass types". It made me question if you have ever used Tangela because Ferro and Tangela do not do the same thing. Tangela is a physical wall who is able to pivot into the majority of our physical attackers like ape, golurk, stoutland being the most important imo and etc and ferro is a hazard setter which lacks Tangela's physical bulk so it is not able to pivot into attacks as well as Tangela but does possess the better defensive typing. Then you mention using victreebel as the better fighting check rather than Tangela and I just sigh at this point. Once again, you cannot compare these pokemon because they are so different. Sure, victreebel is a fighting resist due to its poison typing but when hit with coverage like chan's ice punch, it folds. Tangela is a fighting check because it doesnt take much damage from any of our fighting types then can just switch out.
I think you just miss the point of why people use Tangela and a drop for it when Stoutland still exists in the tier alongside other threatening physical attackers is uncalled for.
I dont have much to say on this, ursa is kinda bad rn and is just outclassed by any normal type above it. What ursa does excel at which it does better than even Stout is stallbreaking. Breaking more defensive teams is the best thing Ursa does because it can just 2hko everything in the tier with its Guts set and isnt actually locked in like Stout is. This niche alone can probably keep ursa around C, it's not D material imo.
"Premier sticky web setter" webs kinda sucks rn but that's beside the point because you're addressing the sd set. You mention the fact that bug/grass can threaten a lot but this mon has a lot of problems. Weaknesses to common typings like Ice, Flying and a rocks weakness doesnt make it a reliable set up sweeper compared to something like Dodrio (rocks weakness ik) but has higher attack, higher speed and stronger STABs.
I also wanted to address your calcs which I find quite irrelevant. Ik you want to show how much Leavanny does at +2 but using ev spreads that dont exist like max defence jelli, max defence mudsdale, 0 defence gurdurr oel and doing calcs on physically frailer mons like z move at +0 like victreebel isnt necessary cuz we all know its gonna kill if its unresisted.
Ursaring: If I wanted a half-decent Normal-type stall breaker, I would just use Zangoose, who, with Adamant, has comparable power to Ursaring (even more so than Stoutland does). The reason I bring up Adamant is because Adamant Zangoose is still notably faster than Adamant/Jolly Ursaring and if I truly wanted a stall breaker, Adamant would be preferred. Additionally, Zangoose has access to Knock Off, allowing it to cripple stuff like Gurdurr and Tangela. The two aren't even in the same ballpark yet Zangoose is B- and Ursaring is literally one subrank lower at C+, which is a travesty to say the least. Nevertheless, it seems that you're content with Ursaring dropping to C or C-, so I'm fine with that.
Leavanny: I'm not sure why you're comparing Leavanny to Dodrio since Leavanny is unranked and Dodrio is all the way up in A. From what I understand, mons that are ranked are ones that have distinct niches in the tier, which Leavanny does. I'm only arguing for this thing to be ranked at all; I'm not saying that this thing should be anywhere close to Dodrio. Leavanny having common weaknesses should not invalidate it since Lycanroc and Absol have an even worse weakness to Fighting, yet they're solid SD sweepers. Again, I'm nomming this mon for <C rank , which I think is more than fair since it is able to threaten so many defensive cores with its STAB coverage. Sticky Web is an optional fourth-slot move as well to demonstrate that Leavanny can provide utility and that it is more than a one-trick pony.