Son of Godzilla:
Another first time watch for me, and goofier than Ebirah. Multiple fights against no-name kaiju-ized mantises and spiders are notable in that Godzilla has no need to preserve them for future sequels.
Sadly, Minilla will return.
Destroy All Monsters
Even Minilla can't ruin this all-star brawl that carries one of the best human-scale subplots in the series. The best 60s Godzilla film.
All Monsters Attack
Minilla's front and center in this most Gamera-adjacent of Godzilla films. Maybe the least essential in the series.
Godzilla vs. Hedorah
After a hit or miss late 60s, GvH moves the genre forward with cutaway animation, innovative kaiju design, and night battles. And no Minilla to be found.
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. Megalon
Goes Gigan one better with a roach rematch and a Godzilla-Ultraman teamup.
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.
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.