pffft why would i need an S
ya know i just went 27/2/7 as 
but riot decided to give a 
4/6/21 their S. Cmon riot really? I even had 188 farm. Your grading system sucks balls.
2 Comments
You are not graded just by KDA, this isn't Call of Duty, nor are your contributions compared to that of your own teammates, or the grades of other champions or in different roles. Instead, a variety of information from the game contributes to your score (think vision, objective control, CS, healing etc.), and your score is compared to other players on the same champion in the same role (there are different strengths, weaknesses, and ways to contribute for everyone after all).
While you may have single handedly won the game through your ability to win teamfights or duels, it's pushing towers and taking the nexus out that is your ultimate objective. Try working on your CS or joining tower pushes when the enemy can't stop you (whether done by outmaneuvering them or through that sick KDA).