Berserk has a lot of things to enjoy, but the one I hear mentioned most often is it's art. The mangaka, Kentaro Miura, has really stood out both to me and many others for that reason. He uses heavy strokes to emphasize certain scenes and he really goes into detail. On top of that, the story is gruesome and action packed as hell. It's rather long, so I recommend reading it a few chapters a day/week to fully understand the story.