CNOT gate
The two-qubit gate that flips the target if the control is one — and the most common way to create entanglement.
Classically it would be a trivial operation. What makes it interesting is when the control qubit is in superposition: the two qubits then end up in a state that cannot be described separately. A Hadamard followed by a CNOT is the standard recipe for producing a Bell state, and those two lines of code effectively open every introduction to quantum programming.
CNOT together with arbitrary single-qubit rotations forms a universal gate set — anything a quantum computer can do can be built from them. On real hardware CNOT is also the expensive operation: its error rate is typically an order of magnitude above single-qubit gates.