If you think about it, Hidan could've easily been the strongest villain in Naruto, even surpassing Kaguya. All he needs is a bit of your blood, as long as you yourself aren't immortal, you're pretty much done for. Mixed with HIS immortality, he's a force to be recokened with BUT-
Hidan's downfall was he was hardly good in any shinobi art. His TaiJutsu/NinJutsu/GenJutsu all bad. He was more of a brawler/widly swing around his scythe kind of ninjas. Say, if he had actually trained in the other ninja arts, actually become S rank in pure SKILL...he could've been the strongest.

Now I know you're gonna say "But he was beaten by shikamaru, he is weak". But that just adds to the point he was horrible in any other type of ninja arts. If he had actual S Rank Tai and Nin, he would've killed shikamaru. Easily.

Now put him against Kaguya and Madara that could say put him in another dimension and boom, he's done. Well true. Putting him in another dimension or just slicing him up in itty bitty pieces would be the only way to beat him and that is technically what he did due to the fact hidan as no strategic mind or skill.

Now imagine a Hidan with a calm head, strategic mind, and actual S rank skill in the shinobi arts. He would no doubt be the strongest.

What do you guys think about this?