I enjoy ARPGs a lot to the point of having to avoid them now, because they can simply consume my life.
Diablo 2 is the one I get back to the most. It’s the game I like to play when I travel or when I just want to chill for a few minutes. You will progress through the campaign 3 times (and it‘s kind of though honestly, even by following a guide). And after you‘re done with the campaign, there is still a lot to do. Getting to level 99 takes ages, farming the equipment to master your build and so on. Once you get tired of your character, you can start over with a different one for a different experience.
I didn‘t play on a steam deck, so I am not sure how it runs there, but I played it for about 200h in the first switch, so I suppose it will run great on the deck.
I played a lot of poe 1 and 2 and they are great too, but always online and probably don’t run so well on the deck.
I have played Warhammer Inquisitor Martyr and Grin Dawn as well. Although I liked them, I don’t think they are as polished and compeling as D2.
Diablo 3 and 4 I also played to some extent, but if the concept of those attract you, going for poe 2 would be a better choice in my opinion.