Showtime has had some of my favorite series in recent years: Homeland, Ray Donovan, The Affair, Shameless, Nurse Jackie. Couple of great recent mini-series too: Escape at Dannemora and The Loudest Voice. For me, it's been one of the best sources of what I would call premium original content, along with HBO.
There are only a couple of Netflix series that rank at that level for me: The Crown and Stranger Things (although there have been several other things that were solid "B"s for me on Netflix). Everyone's taste is different, of course.