Although I have yet to get caught up, there's something about the True Blood opening credits that I absolutely love. This is actually very classic for HBO kinds of openings to have uniquely different and stylish title sequences for their shows. There's loads of imagery and symbolism, the chosen footage to show off the themes of death and rebirth, sin and salvation, highlighting the south by showing the bayous and swamps and the dry heated towns, and even disturbing images like the fox decaying (which is a popular image used in other sources, as I've heard) and also, nudity. It goes by fast in the montage but the nudity is very much used in the title sequence. Even the chosen song itself, "Bad Things" by Jace Everett, really goes together with the entire mood and tone of the show. Since it's a very, very sexy song and it goes along with the vampire/hunger/sex theme that the show often has.
Runner ups: the Carnivale opening title, which uses the same kind of imagery by using real footage from the time period of the Great Depression and religious mysticism and symbolism, the Battlestar Galactica opening title which is very unique since it has a body count of the last remaining survivors of the human fleet and at the very end it gives a quick montages of clips from the episode you're about to view, which hasn't been done on any television series ever I don't think, and lastly is the title sequence for Caprica which does have an HBO/Showtime kind of feel to it; very sleek and stylish.
Masterlist of the days here.
In related news to that, because of the S3 premiere I was directed to this incredibly hot clip which, whoaaaa. O__O Now I understand all the squee that was happening all around my flist. I don't really consider this spoilery because if anything I would show this clip to get anyone hooked onto the show. This prompts me to really catch up on what I've been missing with this show.