Sinister is a character when you write him well, he's very menacing and when you write him poorly, he's an utter trainwreck. I especially hate when they try and write him "funny" because he's absolutely not a funny character. If you were a fan of Madelyn Pryor before she became a constant source of mockery and fanservice, he's especially horrifying. Sinister being a Victorian scientist with all the racism and eugenics that implies works great. However, when you make him ruler of the Universe and pair him with Apocalypse, he gets ridiculous.
Frankly, both Sinister and Apocalypse weren't terribly well written characters from their conception up until their current storylines on Krakoa. Apocalypse's best incarnation was probably in the X-Men Evolution cartoon.
However, Dark Beast I feel like ruins the character of Hank McCoy who has always been one of my favorite X-men. Ever since the Age of Apocalypse, they've acted like he was one step away from becoming a monster when he was always best as the kindest and gentlest of the X-men but for Colossus.
To be fair, Hanks' descent into villainy has been years in the making. Even back in the 60s he was the one who seemed to have more of a chip on his shoulder compared to the others, even Cyclops. He had a "I'm as good as the rest of you, even if you all have better powers than me!" mindset and did somewhat unethical things like testing serums on himself.
Ironically, he was happier and more complete as a character when he WASN'T an X-Man and was an Avenger instead.