I don't think I really got the chance to see Mary Hartman, Mary Hartman. It's partly because it wasn't carried by any of the stations where I was after it went off the air (no reruns). When I was growing up (1980s era, very early 90s), cable was increasingly popular, but many of us hadn't yet started to subscribe, so we had only a handful of local channels, and when we did finally subscribe to cable, even the cable channels weren't nearly as many as they are now.
On top of that, didn't that show have a relatively brief run?
HBO has had several great series: True Blood, The Sopranos, Entourage, In Treatment, Curb Your Enthusiasm. And mini-series: John Adams, Angels in America, Generation Kill.
Speaking of great mini-series, lets not forget: Roots.