Why does Black Widow love sitting on Captain America's shield? Because it's made out of vibranium.

If Captain America's Shield Is Made Out Out Adamantium, What's Hawkeye's Shield Made Out Of? Quicksilver

I think comic book bad guys have the right idea, aiming their weapons directly at Captain America's shield. That's probably his weak point.

If Captain America's shield is vibranium, then... Hawkeye's shield is Quicksilver