"Who plays soccer the best?"
I'm pretty sure that your sentence would be like saying "Who plays the best soccer" vs. "Who plays soccer the best".
吗 is only used for yes/no questions. Since you already have 谁 you don't need another question word.
Nobody says "Who plays soccer the best?" How about "Who is the best soccer player?"
I think the "hen" (very) is superfluous here, since "zui" (most) is already there.