What show(s) do you think has the best opening title sequence? Why?
Since I can't really decide on one, here's a couple.
1). Game of Thrones
The music is fitting, and the visuals are stunning. The brilliantly-executed world-within-a-sphere concept involves cities emerging like clockwork mechanisms, and there are slight variations from episode to episode to indicate changes in the world. While that last bit mostly will appeal to fans, you don't have to even like Game of Thrones (Which I don't think many here do.

2). Warehouse 13
Excellent theme music sets the tone, while visual clues show us what the show is about. Ancient imagery interspersed with shots of the present, and the occasional hint of darkness (Such as blood on the tip of the pen) clue us in to the fact that this show deals with dangerous artifacts. The thematic use of crates clues us in to the storage aspect of the show, with glimpses at the steampunk-style tech Warehouse agents use. All in all, it's an excellent opener. It should be noted that in later seasons, they scaled the running time significantly back to mostly just a title card and opted to show credits during the opening of the show.
3). Battlestar Galactica (2004)
Over the haunting introduction music, we are introduced to a visual recap of the miniseries pilot. We see the 12 colonies (Specifically Caprica), and how they fell. The swearing-in of Laura Roslin, and how humanity was forced to flee the colonies and search for a new home elsewhere. And then a driving gallop kicks in, with short flash-previews of what's to come. It's an excellent introduction to an excellent series.
Honorable mentions:
Babylon 5.
I would have included it here, but I have a hard time picking which season I'd want to use. Also, the intro changes from season to season in pretty significant ways. Ways that are (In season 5's case) kinda spoilery. So I don't want to spoil this for anyone.

The Twilight Zone.
Enough said.