Here's my ratings:
Gojira - 10/10. A complete classic for and it was the movie that started it all.
Godzilla Raids Again - 8/10 It was good, but not one of the best of the Showa series.
King King vs. Godzilla - 6/10. Had it's moments, but it wasn't as exciting as other movies in the Showa series.
Mothra vs. Godzilla - 5/10. I can't stand Mothra so naturally, the movie was boring to me. The Godzilla suit used in it was pretty good, though.
Ghidorah, the Three-Headed Monster - 6/10. Once again, it had its moments, but the story was pretty ridiculous and hard to follow.
Monster Zero - 8/10. I actually enjoyed this one quite a bit because of the scenes on Planet X when Godzilla and Rodan were fighting "Monster Zero". Very fun to watch.
Godzilla vs. The Sea Monster - 6/10. I thought this one was pretty boring compared to the movies that followed it.
Son of Godzilla - 3/10. Boring in a way, and the Godzilla suit used in the movie really sucked, in my opinion.
Destroy All Monsters! - 10/10. One of the best in the series. The movie had some great destruction scenes specifically when Godzilla destroys New York and when Rodan flies over leveling Moscow. And last but not least, the final battle with Ghidorah is an ultimate classic. Great scene with all of Toho's stockade of monsters.
Godzilla's Revenge - 2/10. It had some good scenes but that doesn't help my rating because they weren't from the movie itself. They were stock footage battle shots.
Godzilla vs. Hedorah - 7/10. Very eerie compared to the other movies, but I actually enjoyed this one quite a bit for the dark themes and the battle scenes between Godzilla and the Smog Monster.
Godzilla vs. Gigan - 8/10. I really enjoyed Gigan and the scenes when him and King Ghidorah first appeared. The battle with him and Gigan was absolutely amazing as well.
Godzilla vs. Megalon - 7/10. Another movie that could have been better, but I liked the battle scenes.
Godzilla vs. MechaGodzilla - 6/10. Had some great scenes, but I was beginning to get bored with the alien premise of the series.
Terror of MechaGodzilla - 7/10. Decent, but could have been better.
Godzilla 1985/Godzilla 1984 - 10/10. I enjoyed the dark theme used once again in the movie. The Godzilla suit was awesome as well. Very evil looking, like it wanted to destroy everything that was good as it marched through Tokyo.
Godzilla vs. Biollante 10/10. Once again, great battle scenes and awesome Godzilla suit.
Godzilla vs. King Ghidorah - 10/10. One of if not my favorite of the series, great battle scenes and King Ghidorah looked more kick-ass than before.
Godzilla and Mothra: The Battle for Earth - 10/10. Another classic of the Heisei series. Great new look for Mothra and I absolutely loved Battra. Wished he would have had more screentime than he actually did.
Godzilla vs. MechaGodzilla II - 10/10. Enjoyed it, nothing I didn't like about it.
Godzilla vs. Space Godzilla - 9/10. Great, but it had a lot of simple flaws that could have been don e better like when SG flew through space. I thought that looked really fake and unrealistic.
Godzilla vs. Destroyah - 10/10. Amazing way to end the Heisei series, great effects and another movie with a dark theme that kept you guessing. And Destroyah was great.
Godzilla 2000 - 7/10. Not one of my favorites, but it did have several good scenes like when Godzilla killed Orga.
Godzilla vs. Megaguirus - 6/10. Not one of my favorites and pretty boring until the end scene with Megaguirus and Godzilla.
Godzilla, Mothra and King Ghidorah: Giant Monsters All-Out Attack - 10/10. Great scenes but my criticism in the movie was the Godzilla suit - very stupid looking.
Godzilla Against MechaGodzilla - 6/10. Didn't really like this one and MechaG isn't one of my favorite characters.
Godzilla, Tokyo S.O.S - 5/10. Kinda boring compared to the last one.
Godzilla, Final Wars - 5/10. Not one of my favorites. The movie was supposed to be about the monsters and there were lots of battle scenes but they were way too short. The movie was more about the humans.