Madara.
He had been slowly hyped and built up almost all the way back to Part 1, where his name was dropped in reverence at least once or twice, and certainly since the early days of Part 2. He was a legend and a monster who managed to recreate an eye that had only existed once before in history and cheated death on multiple occasions. He was an actual, legitimate threat who succeeded in killing the two main characters off (at least for a little while) and wiped the floor with almost everyone both while dead and then while not having any eyes whatsoever, including the nine most powerful creatures in the Narutoverse simultaneously.
So he was pretty well-established and he was making some serious headway in the 'threat' department, and things were about to get really intense with a final brawl between himself and Naruto and Sasuke. Everything was in place. Sasuke finally had a villain to whom he could relate to like Naruto had with Nagato/Pain and Obito, as well as a way to "redeem" his clan's name by taking down the guy who was largely responsible for destroying it in the first place. We even had Tenten in the wings with one of the Sage's tools, ready to actually be useful for once and seal him when it came down to it. Shit was getting real.
But then Kaguya happened, a woman who had been mentioned only *once* before, maybe ~30 chapters before she ganked Madara's body? Something like that? Mentioned offhandedly in a Madara lecture to Hashirama. A character with no goals, no build up, no personality, no history in the series to speak of, just waltzing on in and acting like she owned the place by way of Black Zetsu of all people. So she comes in, knocks down the guy who'd been pretty well established as the villain of the series for almost half its run, uses his body as her host and takes everyone to another dimension. And then another. And another. And that's most of what she did, along with a Morning Peacock clone and a technique that could've belonged to Kimimaro, which was her only really threatening move.
But she wasted all those abilities with a stunning lack of intelligence and personality. Black Zetsu had more personality than her and did most of the thinking on her behalf; almost everything she did that even remotely posed a threat to Naruto and co. was initially Black Zetsu's suggestion. But there was no real threatening aura to her, for me. I had no idea who she was and tbh I couldn't really care because for the first couple of weeks I felt cheated out of a fight that actually could've been interesting, and then after that there was just nothing really to her worth paying much mind to. She was a mother who felt betrayed by her sons because she went batshit and wanted ALL the chakra, big whoop. Why was I supposed to care? She didn't fit in with the story's narrative at all and STILL doesn't.
Kaguya COULD have been interesting, yes, but Kishimoto rushed her horrendously. He probably came up with her at the last second and then tried to forcibly jam her in there because he thought she was neat, using Black Zetsu to retroactively screw up his entire story and give her some sort of precedence when she honestly had none. She should've been saved for his next manga where she could've been built up and given depth and explored and maybe be an actual threat. Madara was perfectly fine as a final villain and a foil to the two main characters and would've been a perfect place to end the manga.
And hopefully, with Kaguya gone and him still living, he still will be, because if it ends like this, then damn was that a shitty way for this manga to go out. It'd still be marginally better than how Reborn ended, though.