Best treatment:
All of the characters suffered and the last BtVS seasons were hard on all them - many were partially "character assassinated" but there were some characters that clearly had the writers attention.
Spike: the writers got so invested in him that they forgot not all dialogue/storylines were supposed to be about him.
Wesley: the writers were definitely very invested in his story and in the character, which is why he got so many brilliant storylines and one of the best character arcs ever.
Faith: I think she was beloved by everyone, which is why despite ED unavailability to play the character full time she still had some kickass storylines and made it to the very end of the show.
Harmony: it seemed like the writers really liked her character because she kept being squeezed in everything and left W&H with a recommendation letter even after betraying the boss.
Andrew: the writers were in love with Tom Lenk (and who can blame them?) and as such the character got a lot of good (bad) material.
Worst treatment:
At first glance, that's 90% of the characters so I'll try to narrow it down.
Buffy: I mean, pain and loss were always part of Buffy's journey, but after season 5, it was just plain harsh. Between Buffy's season 6 and 7, she was resurrected, abused, in a depression, nearly raped, forced to coexist and forgive the rapist, became a metaphorical zombie, etc.
Cordelia: obviously, season 4 and being killed off in season 5 weren't great for Queen C.
Fred: despite being a beloved character and the fact that Joss loved her, the writers were terrible to her. They had her stuck between two men half of the time, and invaded her body with an Old One in the last season. Not good.
Gunn: the writers didn't give a shit about him, to be quite honest.
Lorne: I think they just gave up on the character at a certain point.
Kendra: RIP Kendra
That's my list... interesting to look at the female to male ratio...