No one thinks it's a bad Pokemon, and I'm sure everyone would say it's a better standalone Pokemon than everything the the C ranks, barring maybe Togekiss. Its problem lies in that there is another option, Thundurus-I, that is pretty much always superior. It's very similar to Blissey and Florges, in that it isn't bad by any means, but why use it when something better exists? The answer is situational and otherwise small advantages. Florges only has two, higher Speed and a Grass move, over Sylveon, so it remain unranked. Blissey is harder to wear down than Chansey thanks to Leftovers covering hazard and weather damage. Therefore it's earned the C rank. Pokemon that fall in the C rank are either relatively bad or outclassed, and Thundurus-T is generally the latter. Thundurus-T has two abilities, both of which are better than Thundurus-I's and a much better Speed tier. It's really hard to make up losing 10 Speed and the amazing utility Prankster and Defiant provide. Thundurus-T can do both of the sets you mentioned well. However, its flaw isn't in what it does, it's what it can't do. When you use it, you're giving up 111 Speed and Prankster Taunt and Prankster Thunder Wave. I agree, not every team needs those things, but it's really hard to pass up using Thundurus-I on an offensive team. Really hard. However, when you look at what it can do, you can see it's actually better than Blissey. Agility + Double Dance are both solid sets that handle offense and defense, respectively, relatively flawlessly. Its only flaw is that you can't use Thundurus-I too. It's certainly as good as everything in the C rank and at least on par with the Pokemon in C+. It really should be moved up.
Sorry halc for sounding weird and probably making you say "tell me something I don't know." At first I was planning to disagree with you, after you called Tangrowth, Seismitoed, Banette, Venomoth, and Exploud garbage (seriously they aren't -_-). But, I ended up agreeing with you. Thundy-T outclassed, for the most part, but it's less outclassed than Blissey. Tbh I think Blissey is worse than everything on the list, but no one would ever agree to dropping it -_-
I don't understand how you can say it's outclassed when I just gave two roles that it does better than Thundurus. Nasty Plot Agility is better than Thundurus because when you NP you have more power to actually beat stall (chansey can just stall out Thundurus, not so with Thundurus-T), and when you Agility, you have more immediate power than Thundurus would (although I never uses Agility anyway. Honestly Thundurus and Thundurus-T play completely differently, and I CERTAINLY think it plays better than the Pokemon I brought up. There is little reason Thundurus-T should be in the bottom rank when there are clearly worse Pokemon above it and it has roles that separate it from the only Pokemon you can't use with it.