You might want to be careful how you ask that question here as
Animesuki got a C&D from Kadokawa Pictures which effectively makes it licensed.
Practically speaking ... whichever one you can find will do fairly well. They all had their good and bad moments.
Update 1/12/10: The first season is now licensed and on the shelves from Funimation. Please support the anime you like by buying when you can.