Personally it would be claymore, often hailed as a female centric berserk.
A story about the lowest class of a generation of engineered superhuman woman fighting monsters to save humanity.
The art gets better and better as it goes on, with stunning panels everywhere(example below).
The intrigue struggle and strive the characters go through is a stellar example of simple yet effective storytelling.
Claymore is fantastic. It's one of those series where you get to see the mangaka MASSIVELY grow as both an artist and storyteller through the course of the series.
I used some panels from the series as phone wallpapers for a while.
My pick is Dungeon Meshi (which will surprise no-one who frequents !dungeonmeshi@ani.social).
In contrast to Claymore, Dungeon Meshi starts off fully developed. Having read it, I am near certain that Ryoko Kui knew exactly where she wanted the story to go, what she wanted it to say, and who her characters are, even as she was drawing up page one.
Despite the humorous premise, and at times light-hearted tone, Dungeon Meshi strikes deep and hard at what it means to be alive. And it does so in general. Not specifically, like about love, war, or the like.
It's not even just about food.
It explores what it means to be young. To grow old. To be hungry. To be sated. The be understood or misunderstood. To do good. To do evil. To desire or be disgusted. To exist.
The characters are oceans deep, and the world is as wide as outer space.
And this despite having a core cast of five, a massive supporting cast that only appears in passing, and a story entirely taking place within one single dungeon, and the tiny island on which it is located.
Oh and it's also probably the best fantasy story ever told.