I'd say Whoever gets the best grades (AmE) will be rewarded, won't they? If the candidates were all the same sex I'd use the appropriate singular subject pronoun.
Thank you for this answer. I completely agree with you.
"Whoever gets the best results ..." is also non-AmE.
Note how to write an ellipsis. This is not English: "................".
Please, clarify what you mean because I really don't understand.
Blanks are normally signified by '_______'. Anyway, most people who use freestyle dots are not indicating blanks, so much so that I missed this possibility.
"won't they" is correct.
Are you saying I should use "_______" instead of " ............ ". I really don't understand what you mean, but the two indicate a blank to me.
Please, clarify what you mean so that I can improve my question.
Also, thanks for the most appreciated answer.
As you will have noted from the previous responses, "Whoever will get ..." is incorrect. It should be "Whoever gets ...". Also, there should be no comma after "marks".
Whoever gets the best marks will be rewarded, won't they?
I agree that "Whoever will get ..." is incorrect and I corrected it above and also removed the comma.
Thanks for these remarks. Also, thanks for the most appreciated answer.