So, I main viktor and I've been grade 5 for ages now. for some reason I, despite of carrying games/playing amongst the tops of the game frequently, I NEVER get greater than A+, despite of my teammates with worse or actually not that good stats get S's quite often, and I get them myself with other champs every now and then with again, worse stats than I get with Viktor on daily basis. Here's links to two of my past games that both got some B or something:
a week or so ago I even played 19/4 carry game with over 200 farm and it was only A in the end. It doesn't make any sense since I got both solo kills and win teamfights with often getting quadra's, i'm almost every time the top damage dealer of the game, I win objectives, dragons, barons and utilize my W to both zone players out and stun them for our team.