Carbon — Role and Importance to Life

Hydrogen, because while C is the second most common element in the universe, H is the first.

Oxygen, because it can form a variety of of covalent and ionic bonds, and is necessary for respiration.

Silicon, because, like C, it is able to form a maximum of four bonds per atom, allowing it to create chains and rings.

Fluorine, because it's highly reactive, and therefore very likely to form different types of molecules.