1. Berserk. No explanation needed.
2. Vagabond. No explanation needed.
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3. Friday: Forbidden Tales.
This is the definitive best Manhwa ever. The short story "Eternity" in this Manhwa is the single most disturbing stuff i've read since Berserk's Eclipse Arc.
4. Monster.
Johan Liebert and the manga's atmosphere is amazing.
5. The Ravages of Time
The strategy is unmatched.
6. Ai-Ren
The best romance seinen. It's angsty, nihilistic, and steamy as well. Too bad it's short.
7. Bastard
Thriller within daily life.
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Shonen
1. Eyeshield 21. The last two arcs were lower than the rest.
2. One Piece: Enies Lobby, Arabasta, Skypiea, Arlong Park, Marineford. Five amazing arcs for me.
3. Hunter x Hunter. Chimera Ants. Succession War. Two amazing arcs for me.
3. Gosu (Manhwa). It started amazing, the last arc fights were dragged so much, but it still ends on a high note.
4. Toriko. It started amazing, but ended horribly.