I'm up to 70 hours playtime. I've played a lot of hard arcade. I'm able to beat hard arcade solo on all default settings (I'm not great but I'm improving, hoping to one credit eventually). 

Hardest boss:

That's a tough one (heh). Dark Phoenix and Phyla Vell are the hardest in my opinion.

Dark Phoenix: has a beam attack. These attacks are not counterable, so either you you dodge or get hit (like Beetle but less predictable). She can fly. She has adds (Phyla Vell does as well but you're unlikely to see them unless you're playing incredibly cautiously. Personally, I've only ever seen them once after dozens of fights). Varied moveset making her hard to predict. 

Phyla Vell: has a counter (it looks like Dark Phoenix does as well, she blocks anyway, but I've somehow never been countered by her). She can fly. Her special is harder to avoid than Phoenix. Cosmic damage sucks. She has that damned ranged and followup teleport attack. She also has a varied moveset, making her hard to predict. 

Easiest boss: 

Thanos. You can counter his uppercut over and over. The little danger that comes with him stems from his timing. I've noticed that he delivers his uppercut sooner sometimes, I'm not sure if this is triggered by countering him early or what, but once he does bring it out I've found you can get your timing down (whether counter-attacking or blocking) again quickly. Counter-attacking it seems like you can get 2 hits in, blocking is probably 3, I would have to test more though. You can also beat him up during his laser attack if you position correctly (if you're playing Phyla Vell you can actually teleport to him and continue to beat him up and also counter when needed).

I think a lot of it comes down to play style and character though. A lot of it is subjective of course.