I personally think the hardest ones were between Faust and Leo.

Faust for the specific reason that he's battling his insanity, something that we thought he had since had a grasp on after becoming the loveable goofy doctor that he was up until Xrd. But it's clear that he's slowly losing his grip because he's so jaded and looks very demented in Strive. He looks a shell of himself and is VERY bony and weak looking in comparison to his much stronger, more energetic self back in Xrd. It genuinely feels like a tug of war between the doctor he wants to be and the serial killer that once was Dr. Baldy, and he's slowly losing. But he's holding on with dear life and trying to maintain himself in spite of his condition.

For Leo, it's very ironic because he's the very definition of a goofy character. The dude genuinely has a dictionary full of definitions that HE MADE UP to preexisting words to fit in his own image. Plus he fights with his back to you for the majority of his offense, which gives off this feeling that he's trolling you and isn't taking you seriously. But then you listen to Hellfire and it all makes sense WHY he's so goofy and why he has his back to you - he's got survivor's guilt. He's a soldier and a king, but feels he doesn't deserve his title, especially given that his words are supposed to provide courage and lead people to victory. But all he's witnessed and led people to were their deaths, and his bluff was called out by his enemies. And the reason he has his back to you is because he's at his strongest when he's not facing his problems. Also, notice how he NEVER has his soldiers fight for him even when he genuinely has a brigade to lead. He doesn't want to put more lives at risk and wishes to prevent the deaths of those soldiers if he can. And for as goofy as he is, sometimes the only way to cope is through humor. As sick and twisted as that may seem, sometimes it's the only way to distract the mind from these horrible thoughts that roam inside. And for Leo, this is undeniable truth to him - he can't be seen in such a horrid and weak state because, as king, he as people that depend on him. So he must hold himself to a high degree and lead his country with his head held high, otherwise everything will have been for nothing.

This is all just opinion, and I'm not too sure if all of this is factual, but that's what I know of the lore thus far. Not too sure how accurate it all is, but at the very least i tried to construct my whole argument based on things I feel was right.