The folks at TrekMovie have put together a top ten list of best Star Trek characters in costume.
#1 is, justifiably, the DS9
crew in TOS uniforms in "Trials and Tribble-ations," with Kirk and Spock as gangsters in "A Piece of the Action" second, but there are several glaring omissions: the Dixon Hill holodeck scenarios (especially Beverly in that magnificent outfit in "The Big Goodbye"), the Sherlock Holmes holodeck scenarios (particularly "Elementary, Dear Data," which not only has Data as Holmes, La Forge as Watson, and Pulaski in period dress, but also Picard in a top hat and tails, and Worf in a tux), the DS9
crew in baseball uniforms in "Take Me Out to the Holosuite," the DS9
crew in James-Bond-movie-style outfits in "Our Man Bashir" (while this might be discounted due to the crew not choosing
to get into costume, by which they disqualified "QPid," though it did get an honorable mention, it should be pointed out that Bashir and Garak were doing it willingly, and both of them looked spectacularly cool in tuxedoes), and the Voyager
crew in contemporary 1990s garb in "Future's End" (if for no other reason than Tuvok in a do-rag).
Making the above worse is that they did include, for reasons passing understanding, the Enterprise
crew as Akaali in "Civilization" (buh???), Kirk as a Romulan in "The Enterprise
Incident" (which was just silly, and not nearly as cool as Kirk and Spock dressed as Nazis in "Patterns of Force," which also didn't make the cut), and Riker and Troi as Mintakans in "Who Watches the Watchers?" (double buh???).