vs. whoever:
1) Use the -ever suffix when who or whom can fit into two clauses in the sentence, e.g.
Give it to whoever/whomever asks for it first.
Give it to him. He asks for it first.
2) To determine whether to use whoever or whomever, use these rules:
him + he = whoever
him + him = whomever
e.g.
We will hire whoever/whomever you recommend.
We will hire him. You recommend him.
him + him = whomever
We will hire whoever/whomever is most qualified.
We will hire him. He is most qualified.
him + he = whoever