Godzilla vs. Gigan
I last saw this in the theater as a child, and while the cockroach aliens are far less creepy now, the 70s films continue to improve. Anguirius gets a hero turn in the Gigan/Ghidorah tag-team match that also teases the Mechagodzilla. Plus the big G gets a few lines of intelligible, manga-balloon speech.
Godzilla vs. Mechagodzilla
Threepeats are hard! After two improvements on the strong Hedorah film, Fukuda falters just enough to miss on the surefire Mechagodzilla concept. The reveal fight is good, the car is awesome, but 12 years in, the latter Showa tone is starting to wear thin.
The Terror of Mechagodzilla
The latter Showa films underwhelm with a titular kaiju's introduction, but then slay the following film. No so much here, where a guileless Mechagodzilla just goes through the motions.
As is director Honda, who's been on a slide since the stellar Destroy All Monsters 7 years before. A middling exit to Godzilla's salad days.