Hard to tell which one was the first here. This genre is vast, and the limit between an Action-RPG and an Action-Adventure game can be tricky to draw. There's even disparities from a country to another, especially with the hack and slash genre. In France, we use "hack and slash" in the same way we could use "Diablo-like", while this genre is used for a broader spectrum of games worldwide.
Anyhow, Path of Exile is the game i played the most in this genre. Despite its defaults, some inherent to the genre, the way they're maintaining it makes me come back to it every few months. I particularly like the amount of theorycrafting you can do, the diversity and complexity the game has. There's always something new to discover. In a completely different approach, Diablo 3 was also a hack and slash i experienced for a while. But some design choices there don't fit my tastes. Like how itemization works with item sets, or how difficulty is scaled.
Next up, Dark Souls. When i was talking about inherent defaults to the hack and slash genre, one i had in mind is how, most of the time, good or needed items are too rare (and it's especially true in the online variant as they've to keep a relatively stable economy). That's a default Dark Souls obviously doesn't have. And that's also why i tend to prefer such progression, with a less crazy variance between the average build and the best. I think it also encompasses my idea of what a good challenge should be.
To conclude on this genre, i'll name Ys: The Oath in Felghana, an Action/Adventure/RPG. I still remember how dynamic i found the game the first time i played it, with the challenge mainly relying on bosses, and a decent amount of exploration. Ys Origin is also up there with different playable characters to change a bit the gameplay. Nihon Falcom keeps on evolving the series game after game, moving away from the single controllable character in the latest ones.
Note: The listed games are those which spontaneously come to mind. The purpose is to give you an insight of what i like the most in this genre.
Credits: Path of Exile fanart by @yy62401, Path of Exile by Grinding Gear Games, Ys series by Nihon Falcom.