Excluding the tutorials as Ashe, I played Sivir (who was free that week) until I could afford Janna before the end of my first day of playing.
Oddly enough I didn't play tooo many bot games and already was trying PvP on day 1... I recall getting destroyed by a Kat several times so I bought a Thornmail, which I later learned didn't help. ;p
Janna has always been my main and favourite champion since I started playing at the beginning of 2012... All of her skins have gone on sale this year so I have all of them, and well, I suddenly started playing ranked after S2's rewards were announced... And thankfully I can continue to claim that I have all Janna skins, thanks to a 7-game win streak playing my second-favourite champion, who was the first purchase I've made with RP.