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.
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.