I've always been fond of the soundtrack for Monster Zero. It's a really strong and complete score by Akira Ifukube. It's quinessential, in a word. But really, all of the Maestro's music is great. It's hard to judge one against the other. The man was a genius. Masaro Sato was great too. I really dig his music for Godzilla vs. The Sea Monster, Son of Godzilla and Godzilla vs. The Cosmic Monster. And that's not saying anything about his great work in Kurosawa's films.
If I had to pick one Ifukube score as my favorite (damn-near impossible to do), it would be Monster Zero. And for Masaro Sato, Son of Godzilla takes the taco.