Which grade is the best and worst?

I’m confused. I have arks that are grade A and get more bonus turns than my S grade arks. Which grade is better?

S - it has a higher chance

It goes from best to worst: S, A, B, C, D, E

There is still a lot of randomness about the bonus turns, hence why you might see an A get more bonus attacks than an S.  But overall, S should do a lot better over time.