The whole 'leveling' concept has basically become its own subgenre at this point, which is wild. For a starting point, it's hard to beat 'Solo Leveling'. Yeah, it's the obvious answer, but that opening arc where Sung Jin-Woo is the weakest hunter and then gets the System is just perfectly executed wish-fulfillment. The art is phenomenal, especially in the big fights. It does start to feel a bit repetitive after a while—like, how many times can one dude get more powerful?—but for pure, undiluted power fantasy, it's the blueprint.
If you want something with a bit more world-building and a different flavor, 'The Beginning After the End' is excellent. It's more of a reincarnation/progression fantasy blend. The main character's past-life knowledge gives him a huge edge, but the emotional stakes feel higher, especially with his family. The magic system is clearer than a lot of these series, and the pacing from weak child to someone with real agency is satisfying in a way pure action series sometimes miss.