A very good question!
And good questions are sometimes hard to answer.
"Semelyek" definitely sounds wrong. That is a non-existent word, as far as I know.
"Semelyikek" would be the choice, of the two. But I do not like that choice, either!
The real ideal answer would probably be "Egyik sem."
Or, as a second choice: "Semelyik." A simple, singular answer.