Soukyuu no Fafner is awesome and worth a watch if only because Soushi/Kazuki is as canon as it ever gets but annoying nitpick! It's made by XEBEC, not Sunrise. Just shared character designers with G.Seed is all.

I really love Fafner, though, moreso than G.Seed. And I also love Ryvius. XD Both are pretty refreshing takes on the whole 'youth+mech' shenanigan, similar in some ways and very different in others.

*shuts up now*