DS9: Trials and Tribbleations
I'm not a DS9 fan, at all, but I like this episode because it crosses over with TOS, my favorite Trek series. I loved the bit with Worf refusing to explain the strange appearance of the 23rd century Klingons to O'Brien, Odo and Bashir, but Dax was a little annoying, behaving like a teenager through most of the episode.
Would have loved to see O'Brien "meet" Scotty the same way that Sisko met Kirk, but then again Scotty is still alive and well in the 24th century, so anything is possible.