My choice would be Best of Both Worlds. All Good Things would be a close second.
What say you?
I don't think Birthright needed to be a two parter. The two parts had nothing to do with each other. There was no DS9, or Bashir, or Data/Dr Soong stuff in part 2. They should have just condensed that into one episode, and condensed the Worf stuff into its own episode.