I'm about to say something real dumb that I doubt people will believe, but Dark Souls. Any of the Soulsborne games really.
I like keeping my mind stimulated and continuously thinking otherwise it strays off into dark places. Dark Souls, particularly its offline aspect, but sometimes online as well, is hard, but fair. That fairness makes me trust that every time I fail is because of something I did wrong, and not because I was wronged, meaning I can learn from my mistake and correct it.
It always has a sense of accomplishment even when beating the same boss for the 10th, because every time I try something new, and if you don't give it the respect they deserve they still kick your ass.
Just the whole packet that makes me feel like I'm in control, that there is very little to no luck involved, and that it keeps me thinking and always on my toes makes it always relaxing when I play it. Even more so if I have a newbie friend that I'm joining along his first ride.