One of the Big Three of Seinen Manga, by Kentaro Miura.
I first encountered the series in the early 2000s as a young teen, in its 1997 anime adaptation. I dismissed it at the time, thinking it was nothing special, watching Rurouni Kenshin and other shonen series instead.
As an adult in my 30s, I bought the first three Deluxe Editions, and have read the first two, each in around a day. There's so much happening in every cell, with every character, every interaction, and I seem to find more with every look. Like a lot of other "dark" media, Berserk chooses to mock predictable tropes such as the Hero's Journey by distorting and disfiguring them.
Other people's thoughts
A fan-made group called Studio Taka made absolutely outstanding motion comics of the first four chapters. They're extremely well voiced and in some ways are better to watch than the anime: Berserk the Motion Comic
Studio Taka also made some amazing lore videos, with a lovely female narrator.
Super Eyepatch Wolf made a 1 hour 51 minute video. I haven't watched the whole thing. Most notably he points out that a mostly unchanged artwork from Hieronymus Bosch is in the manga, and artist Kentaro Miura says Berserk was strongly influnced by Bosch.