If you were to look at my most played games on steam, Warhammer: Vermintide 2 would be my favorite game, which I don't really disagree with. It's an extremely satisfying hack and slash game, with enough progression and missions to keep me coming back. The five separate characters with their three to four different classes all offering different playstyles also keeps me enticed. I'm in love with making different builds, playstyles that can change depending on how you change the build. Fatshark, the developers of Vermintide 2, just recently released a free content drop, so I am definitely going to put more hours in it soon. If you want a fun hack and slash where you kill giant rats, play Warhammer: Vermintide 2.
Nioh 2 is another game with difficulty that is not for the faint of heart. Out of all the souls-like games I've played (souls-like being a subgenre that just means difficult games similar to Dark Souls), this is definitely my favorite by far. It actually has a story that you can follow, which is rare with souls-like games, and the combat and progression is unmatched. I was astounded by how much variability in playstyles there was in the game. If you want a difficult game with great combat that you can play with your friends easily, play Nioh 2.
Deep Rock Galactic is a fun game that is somewhat like Vermintide. However, it definitely has it's own thing going on. It has no story, but it does have great map and sound design along with satisfying gunplay. Along with the four different classes you can play and the ways to both improve and change your character, it can hold your attention for quite some time. If you want to live out your fantasies from the movie Star Troopers with your friends but as dwarves, then play Deep Rock Galactic.