HBO. Its original programming are way ahead of anybody else's including Showtime.
Rome was REALLY beautiful in HD, Sopranos and Sex and The City was far better than any other similar-type series. I think Six Feet Under, Carnivores, Deadwood or Entourage were totally crap but others seemed to like these too, so don't take my words only.
For example I think Curb Your Enthusiasm is definitely a piece of sh*t but that doesn't mean you won't like it moreover doesn't mean it isn't better than Weed or anything similar Showtime can offer.
Critical comedy shows and series are also much better on HBO: show me anything on Showtime which at least comes close to Real Time with Bill Maher or Da Ali G Show! Leguizamo's, Chris Rock's, Whoopi Goldberg's or Carlin's stand ups are also the quintessences of HBO's comedy lineup. Of course, there are horrible piece of crap shows like P. Diddy Presents The Bad Boys of Comedy but fortunately only a few makes it to the screen.
HBO's thought-provoking minis are also exceptional: remember Angels in America?
Or other controversial matters: their docus like Born Into Brothels or And Along Came A Spider or Balseros or Blind Spot: Hitler's Secretary - heck, AFAIK HBO was the only channel to air the shocking Chernobyl Heart...
HBO. Hands down.
I hate HBO for its really retarded Hollywood-based movie lineup - most of the movies are pathetically sh!tty - but its own programming saves the entire channel in my eyes. Shortly: nothing comes close as of today.
