It's somewhat curious that which of these are "hardest" seems to depend on which part of the country (or which country) was the place where you learned to speak. A full analysis, of course, would need to consider several other factors.

When I came across a "tongue twister" book in Jr Hi School, it was noted that for some reason most of the boys had little trouble with one classic that seemed almost impossible for the girls to get out -

"Rubber baby buggy bumpers"

(as previously posted several times).

The one that stopped all of the guys, and most of the gals was -

"Six slick slim sycamore saplings."

(There is an "extended version" of "saplings," but the short form was usually enough.)

It was apparent, though, that the gals had less trouble than the guys with the saplings.

We agreed then that "buggy bumpers" was a "lip locker" while "slycamore slaplings" was more a "tongue tangler," and actually did some "learned discourse" on why the differences might occur. (Remember, we were only about 13 y.o. at the time.)

Anyone caring to venture opinions on why there would be a difference in male/female abilities here should avoid overly trite jokes about customary uses of the lips vs the tongue, of course. We covered most of them - at pre-teen level anyway.

John