Capaldi's first season was a muddled mess, but I thought the stuff introducing Missy (the best mystery that Doctor Who has had, IMO, although not a big mystery in terms of actual mystery shows), the Danny Pink storyline (finally doing a deep dive into the high price of being a companion that had been hinted at throughout NuWho, but never spelled out) and then the year with Missy as a companion were about the best writing the series has ever had.
It definitely all benefitted from having serious actors like Capaldi and Michelle Gomez to elevate it, but I felt these arcs achieved heights that the show rarely has been able to, and even then, usually only for a single episode. (There are a lot of great Tennant and Smith episodes, for instance, but they're usually book-ended by cringeworthy ones.)