I will make no bones that the basis of my recommendation for "Symbiosis" is that it features Merritt Butrick.
Who is Merritt Butrick and why should you care? He played Kirk's son David Marcus in The Wrath of Khan and The Search for Spock, and as such is one of the more notable actors in the original series movies. Part of the reason I like that he also appeared in Next Generation (though in a totally unrelated role) is that it still seems to me that the fact that Kirk had a son is still pretty unappreciated by Star Trek fans. Butrick himself was not hugely featured in either of his movie appearances, playing at most second fiddle both times, even though he was a capable enough actor. "Symbiosis" then becomes that much more significant, a spotlight of a different kind, not nearly as important, but featuring more breathing space for the actor, who died in 1989 from AIDS.
Otherwise this was the series at one of its early attempts at a "message" episode, something the original series helped form its reputation around and in fact became a tradition throughout the franchise. The message in "Symbiosis" is that drugs are bad. I tend to agree, but the message is delivered with about as much subtlety as painting an alien half white and half black, and his persecutor the same way but in reverse (that happened in "Let That Be Your Last Battlefield"). Although it does give Yar a rare chance to be memorable as she discusses the subject with Wesley (and thus all young viewers).
Speaking of Yar, this is her last appearance, technically. It was shot after the episode that follows, "Skin of Evil," which features her death. Denise Crosby appears in the background waving goodbye at one point!
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Memory Alpha summary.