What can I say? I love them all! They all get my vote. There is one though whom I have a small issue with...the original. This is only because I look back at the design and see how limiting the budget was, having less detail and things of that nature. Still, the original is wonderful and the prop was huge compared with Godzilla in "Godzilla vs. Mothra."
The GMK Mothra and Mothra Leo (Rainbow Form) are both tied as the most beautiful version of Mothra, although with Leo being a "guy" I should say he looks handsome. GMK Mothra looked sleeker and deadly, and still maintained that majestic quality of Mothra. The purple-tint eyes were also a nice touch from Kaneko. Leo (Rainbow) was very colorful to say the least, especially the wings which would make a leprechaun tap dance and hide his gold. Leo's design also made him look almost like a hornet, as mentioned up there about the GMK Mothra.
All in all, have love for all versions of Mothra females and the single, and still living, male. Now if the topic was favorite or worst Mothra story...then that would be a discussion.
Godbless and be cool!