I have to go with In the Pale Moonlight.
City on the Edge of Forever would be my second choice. I think that, sadly for TNG, BOBW has lost some of it's punch since the Borg were so overdone later on...and showed so many times on VOY to be so easily beaten. Because of that, I now watch this episode with more of a shrug than a gasp.
The Borg were definitely a race where less would have been more, IMO.