I would add to the previous recommendations:
must see:
- The Americans (old school/cold war undercover spy series)
- Fargo (inspired from the movie)
- True Detective (season 1)
- Mr. Robot
- Southland (hyper realistic cop show)
- The Killing (the original Danish version, all the seasons)
- House of Cards (US version with Spacey, season 1 & 2)
- Louie (with Louis CK, the new Seinfeld IMHO)
- Utopia
- Orange is the New Black
- Rectify
optional:
- The Leftovers (very intriguing series)
- Suits
- The Good Wife
- Strike back (bad ass British spec ops operatives)
- The Knick
- American Horror Story (for a VERY adult audience)
- The Wire
- Mad Men
- Peaky Blinders
- Justified
- Gomorrah (the Italian series from the movie)
- Borgen (Danish show)
- Silicon valley
But if there's only one show to watch, then I'd choose Breaking Bad.