Toho Kingdom

Megaguirus. She is fast, aggressive and has a wicked design. Yeah, she dies very easily, but her speed really gave her the advantage against Godzilla.

I'd have to go with Keizer Ghidorah, but I loved all three.

Orga for me. The best "clone" Godzilla has ever had with a unique look and characteristics to him. Beyond anything else I enjoyed the build up to him between the UFO and Orga's final reveal.

Monster X specifically. I love his design.

Megaguirus. I loved the design and the way she is portrayed. She's fast, vicious, and fights dirty.

Orga. How often do we have a kaiju that's also a mad scientist and a mindhive?

Orga I feel pulled off the Godzilla/Alien hybrid idea way better than SpaceGodzilla ever did (Plus with a less complicated and more engaging origin to boot)
The idea of an Alien race trying to remake themselves through Godzilla’s regenerative DNA only to pay for their hubris and rash actions with a horrific transformation is almost tragic in a way

Monster X is my favorite original Millennium monster, but I really hate Kaizer Ghidorah. Orga I find pretty bland, which is a shame because it had some amazing concept art designs. Megaguirus is the perfect antagonist for a Rodan film that will never be made.

My vote goes to Monster X. I feel that he is not talked about often but I remember when the design was revealed before the movie was released in 2004 I was hyped. In the movie itself I was pretty impressed by him and how he could go toe for toe with Godzilla for most of the battle. The transformation scene is cool too; however, Kaiser Ghidorah leaves a lot to be desired. Personally, I think the European style dragon design was done better with Desghidorab and tbh Desghidorah might make more sense as an evolution of Monster X than Kaiser Ghidorah himself. I like the size of Kaiser Ghidorah and how he towers over Godzilla along with abilities, but otherwise I was never impressed.

Orga is my boy. Really cool design and execution.

Orga, really love that dude