Why lithium is least reactive in water among group one elements though it has the most negative Electrode Potential value?
Potentials only tell you about thermodynamic stabilities. They do not tell you anything about the kinetics of the system, which would correspond to how quickly the system reacts.
Lithium is the least reactive because it is the one with the least electrons. That means the electrons are closer to the nucleus and therefore more attracted to it. AS you go down the elements in group 1 the electrons are able to react more violently with water because less energy is needed for the electron to leave the metal.