actually, I thought it was worst of the original series, pre 1970 anyway
Sure, vs. Gigan, vs. Mechagodzilla, Revenge, and Son of... are all far, far worse, but as a huge fan of Godzilla vs. Mothra, Monster Zero, and King Kong vs. Godzilla, even Godzilla vs. the Sea Monster, etc., I still find this one a major letdown, and the worst of Ishiro Honda's giant monster movies.
Reasons:
1) Not enough monster action until near the end, and then it's done way too goofily. Slapstick.
2) stupid plot involving political assassins out to kill a princess who thinks she's from Mars. Totally moronic, and it hogs the spotlight.
3) Introduced the concept of talking monsters
4) Introduced the idea of "Good" Godzilla
5) Takes until at least 30 minutes in before any monsters show up, then it's still pretty sparse until near the end.
6) Absolutely ZERO military response to any of the monsters. No scene where Godzilla stomps on tanks or where Rodan knocks jets out of the air.
7) Godzilla and Rodan's suits both are the worst yet. Godzilla looks like Cookie Monster and Rodan's head/beak is too short and stubby.
8) "Get out of my way or I'll call the police!" ... "Don't you know that I AM the police?"
There's still a few scenes I love though, like Ghidorah destroying the city, and Godzilla and Rodan's appearances. I still laugh out loud when Godzilla destroys the boat - it actually honks at him as though to tell him to "Get out of the way!".