I know they're basically all important but what would you say is the most important clotting factor to have in the blood?

Meaningless question: If you were given this as a homework assignment, contact the teacher's supervisor. "Which is more important, your car's motor or chassis or steering system?" Fibrinogen actually forms the clot. This may be what the teacher wants you to say. It will only be activated by thrombin, derived from prothrombin. There are a host of others, many of which are lethal if genetically absent.