"Whom," because it functions as the direct object of the verb "impress."

Although American language seems to be replacing all instances of "whom" with "who." I don't like this trend, but a mistaken "who" is a lot more acceptable than a "whom" when one should have a "who":

"I'll go to the mall with whomever wants to come."

Yikes!

Gemini6Ice, your neighborhood grammar gestapo.