I think when it comes to scaling, I think Titanosaurus would make for an interesting figure. This is because he is significantly taller than Godzilla 64, and would probably scale nicely against Millennium and Heisei figures.
Heisei Ghidorah has a decent design, though I think Grand Ghidorah outshines it in every aspect in terms of design. Its visually closer to Showa Ghidorah's crown of horns.
I guess what annoys me of Heisei Ghidorah is turning his otherworldly cackle into a generic remixed Rodan Roar.
http://dai.ly/x26i278Ah, much nicer.