I think Band of Brothers is the best thing HBO has ever done.
But this poll isn't fair to Game of Thrones since it's not over yet and could get even better depending on how they wrap it up.
Deadwood was great though, same with Boardwalk Empire.
Out of the shows i've seen,
Band of Brothers
Game of Thrones
Deadwood
The Sopranos/The Wire (hard to pick)
Boardwalk Empire
True Detective
John Adams
East Bound and Down
Mr Show
The Leftovers
Never saw Oz but hear it was good. Only saw the Wire once, have been meaning to go back and watch again. Never finished Vinyl, it wasn't as good as I thought it would be but might go back and finish it at some point. Treme is another that i heard was good but never saw. I tried to watch Six Feet under once but didn't care for it, perhaps will have to try again someday. Never saw Rome but if it's on the level of Spartacus I will have to check it out.
Also a big fan of Real Time with Bill Maher and Last Week Tonight with John Oliver. Vice was good but stopped watching it after they interviewed that ******** Marty shkreli.