My opinions:
Happiest
Freedom. It's very happy. Infinite is Strato's happiest album and this song "leads us into freedom"

Darkest
Maybe it's Destiny. Intro sounds so sad...
And maybe darkest song lyrically it's Götterdämmerdung, even I haven't hear it yet. But if that song tell us about the war and Hitler, that song can't be very happy. Also 4th Reich sounds (lyrically) about Nazis...
And suprising - The Hills Have Eyes - is pretty dark
