It depends what you liked about Dark Souls. Hollow Knight is imo the best game to follow the tone and general experience of Dark Souls. But if you just want tough but fair boss fights, then i can't recommend Furi enough, it's not similar to Dark souls in gameplay but its all bosses that give the same feeling of accomplishment.

Hollow Knight is very heavy on fancy pants boss fights.

La Mulana has loads of them too, and the major bosses are all very impressive, but it's also an unbelievably unforgiving pile of puzzles.