The ions that contribute the most to the membrane potential of ions are Potassium (K+), Sodium (Na+), Chloride (Cl-), and organic anions. 

Potassium ions and organic anions are more highly concentrated inside the neuron at rest. Sodium and chloride ions are more highly concentrated outside the neuron at rest. 

This unequal distribution of ions across the membrane of the neuron is the most significant factor that determines the membrane potential of neurons.