I'd say either ( ) or Takk... is my favorite. Takk was my intro to the band a few years ago.
For whatever reason, though, for as long as I've listened to and have known about them, Sigur Ros have never been a band that I've ever felt a desperate urge to listen to. I've never considered them more than a "good" band, either. They've always just been a band that I enjoy listening to on occasion and never regret when I do. But hey, that could change one day.