For me it was George and Izzy, they just didn't work as a couple. George and Callie didn't work either. George and the nurse from season1 were pretty good though.
I didn't think the Owen/Cristina relationship had much chemistry in the beginning, but it is getting better and better and I think they are in it for the long haul. Burke and Cristina had MAGIC chemistry though, and if by some miracle Burke were to return, the triangle between him and Owen and Cristina would make for great watching LOL