I found this serial utterly entertaining.  Sarah Jane is fun, Harry is a treat, it is good to see the UNIT crew a bit for one last time, and I just love the setting of Tulloch Moor.  It just felt like a nice, tight example of the base under siege trope.  There's lots of memorable characters, not just the returning UNIT figures like Benton and the Brigadier, but also memorable new individuals like the Duke of Forgill.  I think production did a good job bringing the energy of Scotland to the table and from there it is just a nice, consistent experience.  It's easy to enjoy something like this with the Fourth Doctor.  He's just so charming and there's a dynamic to how the show was structured at this time that made Terror of the Zygons a lot of fun to follow.  With it kind of being one of the last stories to feature UNIT for a good, long while, it feels like the end of an era.  So there's something very nostalgic and charming about this story.  The Zygons don't stand out much as monsters here but like everything else here - they do the job.  They very much fill the role of a classic, UNIT-era alien force to a T.  And I think in this case that really works to Terror of the Zygons' benefit.  Sometimes playing it to the formula, but doing a very good job at that structure and showing what works best about it.  A bit underrated, if you ask me.