This was really hard (curse of abundance right) so I made some restrictions. No shows that have already been listed, and I also avoided those that are consistently great (hard to pick among those). What remains is a list of standout seasons from shows with varying quality:
Homeland 2
Hannibal 2
Fargo 1
House 7
Master Of None 2
The Good Fight 2
Billions 3
if I'm limiting this to one season per show, my top ten would be:
LOST Season 4
Twin Peaks The Return
The Wire Season 4
The Sopranos Season 3
The Leftovers Season 2
Game of Thrones Season 4
BSG Season 3
Mad Men Season 4
The Americans Season 4
Hannibal Season 2
I don't have the time to watch too much TV but I gotta mention The Office Season 5. Actually I loved all seasons between 2-7 and can rewatch them anytime when I have 20 minutes.
Other shows that come to mind are:
Black Mirror (Season 3)
Breaking Bad (Seasons 2-5)
Game of Thrones (Seasons 4 and 6)
Friends (Seasons... basically 1-8)
American Vandal (Season 1)
Big Little Lies (Season 1)
The Orville (Season 1)
Fresh Prince of Bel-Air (Season 4)
Batman The Animated Series (Season 1)
The two I want to catch up on are The Crown (my wife loves the royal family) and The Leftovers (people around here say it's really good after season 1)