The 4th saddest moment of the series is Ichigo losing his powers after his battle with Aizen. Seriously, the series could have ended right here and it would have been a sad, but fulfilling ending.
So Aizen is defeated (maybe) and Ichigo is the hero of the series, but after using his final move to win, his powers begin to fade. Urahara warns Ichigo that at some point he will lose all of them, but its hard to project when that will happen. Ichigo brushes it off and continues to fight, but as each battle goes, he begins to get weaker, struggling with even the weakest hollows out there. It’s hard to watch as the top hero of the series begins to fall apart. He just can’t fight anymore and has to rely on others around him to protect him.
This leads to the final time Ichigo sees Rukia. Ichigo is standing in his human clothes talking to Rukia, thanking her for everything. They had become a great team together and gave the fan base so much to watch over the years, but then Rukia fades out of Ichigo’s view. His power is gone and his ability to see the souls including the shinigami, is now gone.
Its such a sad moment because Ichigo was the main guy, the leader of everyone, and now he is just a basic human. Chad and others continued to fight, but Ichigo couldn’t even see the monsters they were facing and could do nothing to help out. It was a very sad moment in the series. He does eventually get his powers back, but seeing Rukia fade away was very sad.