Okay someone explain to me what the requirement to get an s rank with quinn is.

Farm and damage to turrets is also a factor. If one of them is majorly below average in comparison to other/average Quinns at your level of play then your S rank will falter unfortunately.

For supports I'm unsure of how it works, maybe damage tanked or heals or something?

Kda, wards, damage to champs, objectives. Kills are the same as assists, hence your deaths have a bigger impact on score than kills.

Ward, take objectives and CS.

Take her into untraditional lane/role where nobody plays her, destroy towers in the lane or have someone destroy them, then assist with destroying other towers. And here you have high grade.

I once had bad game with MF, but in Toplane and we won - got something around A. I had good game with MF, where I was doing fine, until fed Xin stomped most of the game and we lost, additionally played botlane ADC. Got something around C+. Everytime I think well this was horrible I get high grade, everytime I think I've played decently I get low grade. It's not about skill much just certain numbers and factors and from my experience, I sense turrets falling in your lane have big influence and the fact you are compared among the players of the same lane is confirmed.

i got an S rank on shyvana going top; 6/4/1, took 3 towers, 304 cs, dropped my two wards regularly, still lost the game.

I've gone 18/2/14 on champs like Zyra and Lux, but my cs was less than and didn't take many objectives; no S ranks.

Like others have said, it's more than KDA; turrets, neutral objectives, and farm all matter. The only S+ game i can recall getting was recently on Sivir, where i didn't have a spectacular KDA (pretty good, but not godlike), but i had almost 450 cs.