Is there any data on which champions on get the most and least S rank games?

It should be very consistant as it is how well % you perform on that champion in comparison to other players in that role on that champion.

This is why it is easier to get a S on Velkoz support then it is Velkoz mid. Fewer players play him support and most don't do well.

play urgot/galio/yorick i bet it's easier to get S on those champions than the more popular ones.

Low skill cap champs are more likely to give S ranks, but if you're good with a high skill champ the go with the harder champ.

If you actually wanted a statistic, the champ with the most play rates prob has the highest amount of S ranks

(Ezreal and Thresh have the highest play rates in NA)