There's straw poll on Reddit asking what season people started playing, but all of the comments were of people stating what champ was released when they started. So, I figured I'd ask the boards too.
For me it's Lucian! In fact, the first log in screen I remember seeing is Spirit Guard Udyr. After that little event was over, it switched over to Aatrox for maybe a week until Lucian came out. :)