Damn. I was going to say biogeography and convergent evolution until you posted:
In my opinion, I don't think genetics, atavisms, radiometric dating, fossils or biogeography are very useful evidences, because they are too complicated for creationists to comprehend.
With that stricture, I guess I can't think of anything that would instantly turn on the light for a creationist. It is, in essence, the key problem in the debate: evolutionary theory is relatively simple in concept, but extremely complex in detail. If they don't buy the concept, they're never going to "get" the details. Creo websites with their contentless pablum and rhetorical essays are (to paraphrase myself on another thread), "ever so much easier to swallow".
You can lead a horse to water, but you cannot make him think.