Visual representation is more effective than the witches merely telling Macbeth the prophecies because the visual representations are more daunting and longer lasting.
Think about the last movie you saw. You may remember quips and snappy one-liners every now and then, but when you talk to your friends about it, you're not telling them about the brilliant dialogue (normally, anyway). Most of us spend time talking about the awesome car chase that ended in this huge pile up where this poor yellow Ferrari flipped over the pile and exploded in midair spraying expensive Italian car parts a half a mile in all directions down the road.
In Macbeth, like anything made to be viewed (as that was Shakespeare's intent), we're given visual stimuli to create and maintain intrigue, suspense, and a connection between action and audience. The witches could easily have told Macbeth that no man born of a woman will harm him, but why not show him a ghost-like bloody child on top of it? It's reinforced for him in that way. It'll have a greater affect on Macbeth, and it'll be even more lasting with the revelation that is to come later in the play when Macduff reveals that he was "untimely ripped" from his mother's womb. Then, the image of the bloody child makes perfect sense! We have this connection between the two aspects of the play, and it evokes an emotional response from the audience towards Macbeth.
As for which of these apparitions is the most effective, I would say that it's a toss-up between the bloody child and the small crowned child with the tree in his hand. In the end, I'd probably have to go with the crowned child and the tree because it's more obscure, thereby making it more effective (which could be contradictory, but stay with me). Personally, I think that the prophecy itself is more vague with the whole Birnam Wood coming to Dunsinane Hill bit, and the image used to coincide with it isn't as literal as the others, so you have to think more about it, assuming that's how you choose to define "effective" (something that makes the audeicne take ownership of the action, in this example). If you're considering "effective" as the image that most directly relates to its prophecy, then the armored head, albeit boring, would probably be most effective.
Of course, to play devil's advocate, you could argue that, while stimulating and pleasing for the crowd, none of the images are actually effective because Macbeth doesn't understand or heed any of the warnings, despite their truth. Then again, if you're one of the witches, they're ALL completely effective, because they all accomplish the goal of the witches, which is to confuse Macbeth and further drive him into despair.
It's your choice in the end, though. Just be able to defend your choice with examples from the play, and you'll make your teacher proud. I've given you a couple of links to check out that you might find useful, too.