Yasuo's playrate
Yasuo's playrate was steady at about 30% (still rather high for a game with over 200 champions) and then the preseson patch dropped. His winrate skyrocketed to a near 40%. The winrate went up about 3% (from 51% to 54%) because so many people were playing him and doing well (Shocking /s). His playrate at its peak was 41.7%. That is almost half of all normal SR games played.
Source: http://www.lolking.net/champions/yasuo
Edit: Yasuo is currently the most popular champ in LOL by 3000 games (lee sin being second).