The Great Senin War - Houshin Engi - Easily the greatest war arc in any shonen manga ever. Unlike most war arcs which usually are just a collection of 1 on 1 fights, this one actually felt like a large scale conflict. It had such a powerful and overwhelming villain that seemed almost invincible. Also because it took place in the middle of the series instead of at the end like most war arcs, all the deaths that occurred felt way more impact full due to you having to continue reading the series with so much of the cast gone.
Faudo arc (manga version) - Zatch Bell - A massive arc that had so many twists and turns. Zatch Bell is surprisingly emotional series and is one of the very few manga that could not only make me cry but could make me cry often, and most of those tears were from this arc, so many characters had their books burned in this arc and they all went out in an awesome blaze of glory.
Chapter Black - YuYu Hakusho - After re watching the series recently this arc isn't as good as I remember it, however it still gets a special place on this list due to the effect it had on me when I watch it as a child. This arc was so different and so unorthodox compared to not only other anime that were airing at the time but also compared to what YuYu Hakusho itself had shown previously. It really changed my perception of what exactly a shonen could be and it was the point at which I realized Togashi was truly a cut above the rest.
York Shin arc - Hunter x Hunter - To this day this arc still blows my mind. It was completely unpredictable and plot driven instead of fight driven, which to this day is still pretty rare for a shonen manga. Its like Togashi read every single shonen troupe and decide to subvert each and every single one of them. Everything about this arc was unpredictable from the main characters being almost completely useless, to the anti-climax, to the appearance of the Zoldycks.
Alabasta arc - One Piece - this was the first major arc in One Piece and was the arc were I truly became a fan. Before this arc I thought One piece was pretty generic and formulaic, however the amazing amount of build up that Boroque works had and the equally amazing pay off of the final fights changed my mind. Plus the fact that Luffy lost not just once but twice to crocodile before he scored the win, hyped up the schichibukai so much.
Chunin Exams - Naruto - Even though I pretty much despise the series now, Naruto was the show that had popped my cherry. Before Naruto I had never read a manga before, and the only exposure I had with anime was through toonami/Adult Swim. However this series opened the floodgates to me discovering so much other anime/manga that I probably would've never discover otherwise. This was the arc that made me fall in love with the series and what made it for years, my favorite anime/manga.
Soul Society arc - Bleach - One of the earliest arcs of the series, a series which sadly hasn't produced anything even close to it's quality in since. IMO the definitive rescue arc, I especially love all the intrigue that occurs during the second half when everything starts to fall apart in soul society and nobody has a clue why.
Food festival arc - Toriko - This is the most recent edition to this list. This arc was so good, which was great because the series was in a decline before this arc. This arc set up fights that people had been waiting to see since almost the beginning of the manga and the second half of this arc, shock up the entire series and completely changed the allegiance of so many characters.
- Shadow of the Moon, Sea of Shadow (月の影 影の海
