There's no one best series for me. I have my favourites for different reasons. And while some seasons of them I would consider quite bad, highest on my list overall would be (I'm going to list quite a few, since people are often wondering what to watch):
Sopranos
Breaking Bad
Billions
West Wing
House of Cards
Sons of Anarchy
Vikings
Homeland
The Americans
Band of Brothers
Stargate SG1
Seinfeld
Californication
Boss (sadly canned after one season)
Weeds
How I Met Your Mother
Grace and Frankie
Better Call Saul
Honourable mentions:
Outlander
Game of Thrones
Madam Secretary
The Newsroom
Sense8
Westworld
Turn
Studio 60 on the Sunset Strip (sadly canned after one season)
LilyHammer
Long Way Round & Long Way Down
Masters of Sex
Orange is the New Black (early seasons are definitely better)
Rectify
Terra Nova (sadly canned after one season)
Life in Pieces
Code Black
Black Sails
I'm still yet to watch any of The Wire or Boardwalk Empire but I understand they're meant to be good. Also want to watch The Night Manager, Rescue Me, Narcos, Mr Robot (I started but need to get back to it), The Man in the High Castle, Hell on Wheels, Empire, Deadwood, Bull and Ray Donovan.