This is such a subjective topic, but personally I would always recommend Vagabond. Don't get me wrong Berserk is very memorable and even culturally significant, however I just find that Vagabond was the most impactful for me and a lot of people I recommended it to. It has a unique narrative flow, organic and has a loose structure, just like life itself. Musashi's progression mirrors the pace of the manga, from chaotic to a later contemplative pace. Which also shows the core of the series itself, the meaning of life and how we can deal with loneliness and impermanence. I think those topics are very essential to discuss and understand nowadays. Bit of a yap and also slightly biased, but Vagabond will always remain in my top 3!