Need to make a medieval siege engine for school. Help?

So for Global Studies, I need to make a model siege engine. I was just wondering what the cooler designs were. The torsion catapult, trebuchet, and ballista are what I narrowed my choices down to. Do you have a preference of those? I honestly think the torsion catapult is too simple and the ballista seems very dangerous to bring to class. What do you think would be best?