Most diffusable ion in excitable tissue is -
Points to note
Concentration of ions

Na+ concentration higher in ECF 
K+ concentration higher in ICF 
Cl- concentration higher in ECF
K+ leak channels allow K+ to move from ICF to ECF
Na+ is prevented from entering the cell.
Thus diffusion is highest for K+, then Cl- and least for Na+, when the cell is at rest.
Na+
K+
PO4-
Cl-