There is no "hardest" scramble. There are hardER scrambles, and even then, it's circumstantial. Depending on the method used, a scramble could be easy for one person, and difficult for another.
Actually, wouldn't the hardest scramble be the scramble that would need the most moves if you were to solve the cube without any unnecessary/extra moves ( as in the best shortest solution)? Though if you were to philosophize it. Then we'll have to define what it is for a scramble to be the hardest — or harder than another —.
Now the definition has two parts( as in the definition would be It's the scramble that
First Part: Second part):for the first part, I'd just save time and skip thinking and wondering whether it is the one that takes the longest time to solve or the one that takes the most moves to solve or the one that makes you think the most or the one that makes you feel worst or a combination of those or whatever. I am skipping all that and I am gonna say: It's the scramble that gives you the hardest time (This should suffice for now). Now the second part is the important part (or at least I'd like you to think so to make things easier for me): gives you the hardest time when/what?
1. At the time you try to solve it?
2. If you were to be the best possible for yourself and tried to solve it?( now, anytime, usually...
3. If the you right now tried to solve it right now ?
4. If you tried to solve it anytime.
5. If the best humanly possible tried to solve (tried to solve it now, anytime, usually....)
and the list could go on infinitely — like hell do I care though —. So whichever plausible answer you choose from the long list we are stuck with the maybe(ok fine, most probably) impossible way to find what's the hardest scramble or even which scramble is harder than which, as we will be stuck with all those endless statistics,factors and induction trials or the hunt for a psychic potential, etc. Or if we get an answer like the one that takes you the most moves even if you were to solve using the fewest moves possible, even if you get the answer to that, the scramble won't do you any good unless you were trying to solve with the fewest moves possible and after the first few times you solve it that's it, you will eventually find it very silly and it will cease to do serve the initial purpose intended (go to the second hardest and so on?). Note that the fewest moves is an example, for instance if you use Fridrich then the knowledge of the hardest scramble for Fridrich( the one that would requires most moves and —usually, I guess— therefore most time) would just render the scramble( and the second hardest and the one after...) to be just general knowledge. Though for a speedcuber using Fridrich maybe knowing the exact moves you'll be doing might do less harm and solving the hardest over and over might help practice (then get you too used to it and screw you up with the rest of normal random scrambles). Though unless you are participating for a Fridrich hardest scramble speedcubing competition(Guy1: hah I solved the hardest Fridrich scramble faster than you. Guy2:Hah I solved it faster using Petrus *Killed*) or unless you know that this is the scramble coming in the competition, then the hardest scramble for X, even if both definable and acquirable, might not be of much use except for practice at the beginning only as it will be like solving any other scramble over and over and over.
Gah! I don't wanna think about it anymore. Why did you ask this question? — no it's not another philosophical question — just use a damn scrambler, or go into the great thinking chamber and try to come up with some deductive/inductive/whatever answer, impress us!.. if you really insist XD