Black Swordsman: 7/10
Golden Age: 9/10
Lost Children: 7/10
Conviction: 10/10
Millennium Falcon: 10/10
Sea God: 6/10
Fantasia: 8/10
Black Swordsman and Lost Children are just introductory Arcs for bigger Story Arcs, so their ratings are self explanatory.
In retrospect, the Golden Age Arc is great, but also overrated, because Berserk is at its best when it's about demons, not politics. Conviction is a better Story Arc for Guts than the Golden Age is. I used to rate Millennium Falcon lower, but then I realized it's also Griffith's Story Arc, hence the title, and it's a better Story Arc for Griffith than the Golden Age was, as well. If I stop and think about it, the Golden Age was about both Guts and Griffith, and so is Berserk as a Manga. People occasionally make the mistake of assuming Berserk is only about Guts, but that's just not the case.
Sea God is terrible by Berserk's storytelling, but not by Manga storytelling in general, considering Berserk is one of the best fictional works out there. Fantasia is fine as a Story Arc, I feel like it gets too much criticism just because it can get boring sometimes... LOL.