The one I haven't read, yet. However, that answer is boring. Instead, get a list!
1. Berserk: it's the most beautiful manga series there is. Massive highly detailed sceneries, incredebly atmospheric places, filled with such a vast diversity of creatures ranging from visually pleasing to downright traumatizing encounters.
The fights are also on another level. No silly swinging around of swords like toothpicks. When Guts swings his gib piece of iron you feel the weight down to the last gramm. Though, not only the fights, but the characters are really deep, as well. And given, what a dark place the world of Berserk is, you can see how it affected every one of them. You wouldn't want to trade with them, but watching them in their attempts to struggle through all the danger is kind of inspiring.
2. JoJo: To be honest, the structure of any part (well, starting from part 3 that is), is kinda the same. However, the different settings and diversity of character well makes up for that. We know, it's going to be another game of chess, but the characters and their abilities are so vastly different, you always get left to wonder how things are going to be resolved this time.
3. Hunter x Hunter: I think what makes this one one of my favorite is the intriguing power system. That alongside with every arc feeling so different from those before, while still being believable.
4. One Piece: mostly for the worldbuilding. While usually you see the protagonist at least taking part in any big event that's supposed to change the world they are in, in this series that's actually only sometimes the case. In One Piece a lot of big events happen all over the world that the Straw Hats have no influence in whatsoever. There could be a hugh raid on another island happening that we only hear of after the fact. But it still ends up being a major point in the world, anyway.