CthuluIsSpy
Arcane
I thought you got a chest regardless of whether you were premade with the S-rank guy or not?
Then again I haven't had a game yet where someone on my team other than me got an S
edit:
For real though, keep in mind that the drops are not just for ranked. I guarantee you it is easier to get S's elsewhere. I've gotten 4 or 5 so far just playing a few blind pick games. Probably because my MMR for that queue is bad since I never play it. When rotating game mode queue comes, you can likely pick some up easily there too. At least I've found that the game modes have been very easy to hard-carry in the past. I think they reset the MMR each time and give you randoms to play with.
If you get the S, then yeah, you get a chest.
BUT if someone else has an S, then you will only get a chest if you are premade with him.