LLS Reimu. I have a much harder time with Marisa, yet everyone says Reimu is supposed to be the hard one.
To me, though, it's more a matter of individual boss patterns than bosses as a whole. For example, Satori's Princess Undine card was one of my personal easiest timeouts, yet everyone says it's supposed to be horrifyingly hard; I just don't see the issue with the card. But I can't say Satori as a whole, because I struggle badly with Border of Wave and Particle for some unfathomable reason.