My money's on it being a pomegranate originally. Apples wouldn't have existed in the fertile crescent over 2000 years ago. Pomegranates are also messy and look bloody when eating them, fitting the "carnal knowledge" side of the story. I've heard other people suggest they could have been dates, but pomegranates seem like a way better fit for the story.
Adam and Eve never once complained about how freaking annoying it is to eat a pomegranate because 90% of it is dried bark and every tiny seed has its own inedible seed so I doubt that's what it was.