Nirrti is my choice for the most beautiful (and yet strangely, the most evil) of them!
What Atlantis lack for me is Stargate's linking ancient history with aliens and future technology. That is part of what hooks me into series...beyond stuff blowing up!!
I just started collecting the series (Season 2 on its way), and its fun to see that they really seem to have had a focus and direction from the first season. It's fun to see these shows in order!
Still, they are making a good run of it, and the stories are moving along well.