S rank is supposed to hard to achieve, but lets be realistic.
I'm not someone who lower's the difficulty setting to get through a game, I prefer things on hard to be rewarding.
But when I play a really gratifying game of ARAM, where I am pulling off amazing plays, getting triple kills, and taking two towers by myself with a great stat line to be shown an A at the end of the game, that stings.
To win with a stat line like this: Time: 15:19 Kills: 11 , Deaths: 1, Assists: 11, Creep:78, Gold 10,378, 9 Kill Spree, Triple Kill, 2 Towers Destroyed (by my champ). and be denied is hurtful.
I know every champion has to come by different criteria to make their rank, but I can't imagine that stat line on any character not being recognized as masterful play. Especially in ARAM, where simply not dying is hard enough, but racking up high scores while not dying is incredibly hard.