Paying off from the first season episode "Fusion" as well as touching on real world and fictional topics of great significance, "Stigma" is a standout for any number of reasons.
For one, it's the AIDS episode, which garnered Enterprise a fair amount of publicity at the time, a classic Star Trek social metaphor entry that deals with one of the worst health epidemics of our time. Formerly one of the things the franchise was best known for, I think awareness and appreciation for it diminished a great deal during the period where fans started to embrace other genre properties and leave Star Trek behind, one of the reasons ratings and interest declined to the point of the first cancellation in forty years. For that reason alone, "Stigma" is a wonderful reminder of the potential of the franchise to create drama that's not only interesting but topical.
But keeps it grounded in Star Trek, however, is the episode's Vulcan elements. "Stigma" is a mind meld episode. In the original series mind melds were synonymous with Spock and by extension all Vulcans. I think one of the ways Enterprise offended some fans was by suggesting such a strongly established trait of an entire species was not always such a common or accepted practice of its society. "Stigma" (and the earlier "Fusion") goes so far as to say that the Vulcan mainstream at the time shunned the practice!
So not only is mind melding bad socially but T'Pol has also contracted an illness from it, by the way, and so the episode is about how all of this gets dealt with. It's strong continuity both for the series and franchise. If you're still scratching your head about reconciliation of Vulcan society from one era to the next, there's the trilogy of episodes from the fourth season that addresses that, bringing back a more traditional practice of Surak teachings.
I would go so far as to say as Enterprise Vulcan episodes go, "Stigma" may be the essential one.
And that's not even all there is to say about it! The episode also features a visit from one of Phlox's wives, and so also features the first time we meet another Denobulan. As a bonus complication, this subplot involves Trip once again stumbling through understanding another culture. It works as a commentary to other developments in the episode as well as for the character of Trip and a look at Denobulans and and Phlox, who is always great fun to watch.
franchise * series * essential * character
Memory Alpha summary.