Three roles that were lucky to have won, much less be the "best" of all time.
There have been worse choices made, than those you cite.
Fletcher doesn't get enough credit for her performance as far as I'm concerned. When I consider how much hate and scorn her character can invoke in viewers, compared to what I see as a sweet and kind natured personality that Fletcher appears to project in person, I think she did an admirable job. Admittedly, 75' wasn't a strong year for female lead contenders and it could be argued that Fletcher was support, however, her spiteful and controlling character still left an indelible impression on many film goers.
As for Dunaway, she was fine in her film and I have no qualms about her win, even if it wasn't as natural or as real a performance as what Spacek gave us in CARRIE-76'. Dunaway was working with a satirical and heightened script and was terrific in her own right.
Taylor was also very good in Woolf and while I haven't seen the other nominees, I would say she was a popular choice that was still deserving. I would like to see Lynn Redgrave in 'Georgy Girl', as I have heard good things about her performance and the film.
Exorcist: Christ's power compels you. Cast out, unclean spirit.