Does League of Legends have a tournament that is recognized as the 'Most Important'?

Solution 1:

They have a championship at the end of every season. Last year the season 1 championship was the dreamhack tournament which was won by Fnatic, but we don't know when or where the season 2 championship will take place yet.

Solution 2:

I am not sure if I am answering this correctly, but I know at the end of this season there will be a tournament for 2 million dollars

source:

http://competitive.euw.leagueoflegends.com/competitive/season-2/introduction