Currently, I vote for Gaudreau.
He has a 67 NHLE, while Bennett has a 40 NHLE.
I know that in Gaudreau's draft year he had an NHLE of 25, but until Bennett can hit close/similar to a 67 NHLE I'll say Gaudreau is the top prospect this year.
Also note, NHLE isn't the most accurate measurement of talent/skill. But if the difference is by 27 points it still shows that Gaudreau is currently still a few steps ahead of Bennett.
Another note is Monahan's NHLE when he was drafted was 33 and he scored 34 points this year.
While I agree that Bennett will be a beast in the future, he's currently not the #1 prospect this summer. He'll have the tools to easily be the top prospect in next years summer poll. Unless we draft really high next season.