The first Godzilla movie I ever owned- in 8mm form, that is. It's also one of my favorites, since it's the first time we got to see four giant monsters in one movie. Not to mention a titanic team up of the "Big 3" of Toho. That was needed to defeat the terrible threat of Ghidrah!
It's a pretty impressive movie, though they really give quite a bit a way in the trailer. I'm sure that wasn't so apparent back in 1965, but now... well, it's a great trailer, full of cool shots of all the monsters.