My favourite pairing is NarutoxSasuke - It's the main theme of the real manga, too, you see. And that makes it a little hotter....
(I was going for an evil grin...:P)
My least favourite pairing is any one that has Orochimaru, Kabuto or Kisame in it.
@SilentAlarm23 : I agree with you about OrochimaruxSai ... WTF Seriously???