I think it would have to be either Sydney or Irina, i can't decide! Irina is a very complex, intelligent woman - so interesting to watch on screen, and Lena Olin is a great actress. Sydney is strong, very loyal and you can't help but root for her in whatever she does.
Actually i might do a u-turn and say that Sark (you mis-spelled his name in the poll btw) is my favourite character. He is somewhat predictable in his actions because he always puts himself first, but he is great to watch on-screen and we're getting to know him better this season