The Motion Picture and The Search for Spock are both underrated. And both are better than any of the TNG movies.
Caveat: Special Longer Edition of The Motion Picture. It’s got more character scenes.
I’d argue that The Final Frontier is a tad underappreciated and that’s it’s probably the best of the “bad” Trek films. The TNG films bar First Contact really blow, while I’m not keen on the Kelvin movies. At least The Final Frontier can be used as an argument for auteur theory, what with Shatner presenting “his” vision of the series and his understanding of the characters.