why the electrode potential of the copper is positive while that of other transition metals is negative even when the hydration enthalpy for most of the metals can't counter the sum of enthalpy of atomisation and the 1st and 2nd ionisation enthalpies?
Those metals which don't shed electrons as readily have positions of equilibrium to the right. Their E° values get progressively more positive. Copper forms its ions less readily than hydrogen does. Of the two equilibria :
the hydrogen one lies to the left. That means that there will be less build-up of electrons on the copper than there is on the platinum of the hydrogen electrode. There is less difference between the electrical charges on the two electrodes, so the voltage measured will be less. A major change is that copper is the more positive (less negative) electrode. The voltmeter shows the hydrogen electrode as the negative one and the copper electrode as positive.