For me, it's the opening scene of Voyager's "Macrocosm" where Neelix is negotiating with the Tak-Tak. Whenever I watch that scene, my mind is torn between thinking... "What in the..." and cringing. I know it's a form of communication, but it is bizarre.
Oh, and those C/7 scenes in "Endgame". Chakotay's "I want to make sure it's in transporter range of you" is like being forced to swallow a bucket of sugar. It's just too cheesy and unrealistic for my tastes.