One Piece - War Saga (Sabaody, Amazon Lily, Impel Down, Marineford, Post-War)
That was... really something. It's pretty impressive how much of a page turner One Piece can be when it gets serious. There are so many different things happening at any give time that it's really hard stop reading because you really want to know what happens next.
I think the dramatic curve of the larger arc here was really interesting because it didn't start with much indication that the Whitebeard and Ace events would have much direct connection with what the Straw Hats were doing, especially since they were on their way to the New World, and it would not have been unusual for the larger events in the world to play out without Luffy until later on.
But instead of following a more predictable pattern, the narrative presents several red herrings in the Sabaody arc, making it seem like a foregone conclusion that the characters would head off into the New World shortly. There's the introduction of a new range of rival pirates, the 3 day "time limit", and Luffy repeatedly not wanting to get involved in Ace's business. Then out of nowhere, the crew are completely separated, completely derailing the progress the story has been building up to. Surprises like this are really good, especially when the pay off they lead to is worth it.
What was particularly impressive was how the story was willing to go with a solo Luffy narrative for so long. One arc after another with just Luffy, without his ship, without his crew, and yet he manages to build an army for himself to challenge an enemy far beyond his capabilities. While this might seem initially like a sort of cop out, denying his crew of the glory and development they deserve at this pivotal moment in the story, by the end it becomes extremely clear what the intention of this was.
Luffy has grown throughout the series - he has gotten stronger, he has gotten bolder, and his crew members have increased in both quality and quantity. But has Luffy grown as a leader? Is he even a natural leader? Was it just luck that got him the support and loyalty of his crew? Did he only make it this far because of the crew carrying him? We saw back in Water 7 that Luffy's leadership qualities were definitely there, but he was still dealing with people who were already on his side. With the Impel Down and Marineford arcs, we see how much Luffy has truly grown as a force to be reckoned with in the world. His magnetic personality and willpower draws many people to him, both former foes and strangers alike. It really felt like a special sort of foreshadowing for where the series could go in the far future, if Luffy ever feels the need to create an army for a cause again.
I also think it's pretty shocking how major characters actually died at the end of the saga. The manga has always been so resistant to killing off any character, no matter how badly wounded they get. It's actually something I really enjoyed, because it gives a sense of hope for everyone - both heroes and villains, and getting beaten just means they learn from it and either improve their skills or change their outlook on the world to live a more peaceful life. Having Ace and Whitebeard actually killed, and making Luffy's entire mission just fall apart right in front of him is a pretty powerful sucker punch. I guess it's also the natural motivator for the time skip, so all the characters can get much stronger before they head into the New World.
Oddly enough, even after all that, I'll say that I personally felt the Enies Lobby arc was much better executed. The scale and twists throughout the War Saga is no doubt the highest that One Piece has ever seen, but on a character level, and in terms of pace, Enies Lobby was much more satisfying. Impel Down was a lot of fun in terms of variety, but I did go "reaaaaaaally?" a few times as they kept trying to make the prison seem more and more terrible as it got lower. After a certain point it was just comical and started to lose the effect of horror. Lol. I loved the PX units showing up in Marineford though, that was totally a X-men Sentinel moment. They're probably the best enemies the World Government have come up with so far. (Okay I just love robots, sue me.

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Definitely looking forward to the post-timeskip stuff. I hope they actually have details on how the crew have improved themselves in the two years. Especially Nico Robin. The Dragon plot elements are extremely interesting to me.
P.S. What did the fans reading One Piece weekly think about the chapter where they're all talking to Rayleigh and Luffy basically goes "NO SPOILERS! TELL ME AND I QUIT!" when Ussop was asking about the treasure? I thought that was pretty cool, because it served two purposes - 1) it teases readers with "hey, are you still interested in this larger mystery?", 2) it confirms that the actual direction of story is not so much about what One Piece is, but rather the journey to One Piece. I think it's a good thing to be able to dangle an "answer" in front of the audience and deny it from them, without making people really mad. It means that people are mostly reading the story for the story itself, and not for perceived promises of answers which could "justify" a lackluster ongoing story.