Ghidorah: The Three Headed Monster is one of the crown jewels of the Showa Godzilla series. Aside from introducing Godzilla's titular arch rival, this movie marks the turning point in the franchise, where Godzilla transitions from antagonist to anti-hero. In my opinion it's a huge step up from Mothra vs. Godzilla, which was released prior during the same year.
The monsters are very well-rounded as characters in this film, and are arguably better-developed than the humans. There's even a scene where the monsters *talk* to eachother, and it's as hilarious as it sounds. The action sequences are nicely done, a nice blend of slapstick and straight-foreward monster fighting. Ghidorah is at his best in the film, in stark contrast to the rest of the series where be becomes increasingly pathetic (until eventually wearing out his welcome entirely in GMK.) The puny humans are as dull as ever, but the monsters effectively carry the film.
I got this one as part of the Classic Media box set, and while I found the audio commentary tracks on the rest of the movies rather dry and dull, the audio commentary on this movie is actually rather engaging. The presenter's enthusiasm is infectiuous; this guy should have done the commentary for the rest of them. Still doesn't compare to the Godzilla 2000 commentary though. I prefer actual insight from the filmmakers to trivia from a movie critic.
Ghidorah: The Three-Headed Monster is one of my favourite Godzilla movies, and easily one of my top five, along with Godzilla 2000, Godzilla: Final Wars, Godzilla vs. The Smog Monster and Godzilla vs. Megaguirus. I give it a 5 out of 5.