Sliders/cutters are thrown off the index finger. Sinkers are generally thrown off the middle finger, unless your middle finger happens to be off the ball.
Just because both of these guys are Cy Young candidates for their respective leagues, it doesn’t mean their sinkers or sinker grips are the best.
I would argue that Fausto Carmona and Felix Hernandez have just as good as sinkers as the two mentioned in this thread. Both throw their sinkers in the mid 90’s. Guess what grip they use? Just an ordinary two-seam grip with each finger on a seam. Of course, it’s just a subjective argument and there won’t be any agreement on the matter.
Like, connorwilson said, just use the grip that works for you.