What makes an Ultimate skin isn't the unique skin line
But how it brings such a different visual and enhanced experience of a match. DJ Sona has her songs, Spirit Guard Udyr has his evolving stances, Elementalist Lux has her forms and for Pulsefire Ezreal, his ability to 'upgrade' over time via ranking up his abilities.
At first I thought a unique skin line was a part on creating an ultimate skin line. But after reading a comment from facebook saying how it's the different visuals and effects that makes it Ultimate, any skin line can become an ultimate skin. Props out for Riot for creating amazing skins