1. Griffith sacrifices the Band of the Hawk during the eclipse.
2. I never expected
L to die right in the middle of the series. Having a Shinigami take him out was an even bigger surprise.
3. The Millennium Earl's identity in D. Gray-man was a game-changer for the entire series. So much lore has come from that reveal. Too bad this series is underappreciated and a relative unknown to many.
4. The truth behind how to make a philosopher's stone completely changed Ed and Al's outlook on their goal.
5. Aizen's reveal was doubly deep as not only was he not dead, he was the big bad the whole time; it really came full circle during the Everything but the Rain arc.
Honorable mention: The Promise Neverland - Probably the best hook for a first chapter of almost any media I have ever consumed.
This should have also been on the list. Such an amazing series, and this reveal was so unexpected.