Any talk of best Showtime series should begin with Homeland....I've watched that series from day one, and it just continues to get better...can't wait for season 3.
It goes Homeland, House of Lies, and Nurse Jackie in that order for me.
As for Girls, I watched 3 1/2 episodes, never laughed once, and never watched it again--complete garbage(aside from Alison Williams), IMO.