I went with "Eye of the Beholder." The Lactrans are cool aliens, nicely nonhumanoid, impressively advanced but not falling into the "godlike incorporeal" cliche; and it's nice to see another Starfleet crew, something that rarely happened in TAS (I think this and "The Pirates of Orion" are the only cases). "Ambergris" isn't as appealing; the whole water-breather transformation is rather implausible, and it suffers from the annoying Margaret Armen conceit of alien terminology that's thinly and ridiculously disguised Greek/Latin/English, like "surgo-op" for a surgical operation (not to mention "The Lorelei Signal"'s "opto-aud" for an audiovisual display and "lura-mag" for a device that lured men with a "magnetic" pull).