The only series I can think of with more complex characters is Bodies, a BBC series about an obstetric/gynae unit in an NHS hospital. This is an altogether darker and less 'satisfying' program than Six Feet Under, and harder to watch. It has the most challenging medical procedures I've seen on TV - much more extreme than House or CSI for instance - and I have literally had to switch it off and go and do something else to get the images out of my head. The characters are morally highly ambiguous, and it is hard to form a fixed view of them, just like in real life. The politics and sharp practice is like my real working life, without the literal blood and guts of course. My favourite character is Roger Hurley, played by that smarmy chap from The Office. Hurley is morally ambivalent, ruthless and perhaps lethally incompetent. Kind of love to hate.
Both Six Feet Under and Bodies are sometimes sexually explicit, in a way which would have been literally unthinkable for a TV program even quite recently. Both programs linked the extremes of the body in death and in life, and I never felt that the extremity of the sex was gratuitous or exploitative. Like imagination or suffering it is one part of life, that makes sense of the rest of it.