Somewhere on the Equator and easy enough to travel to, whilst connected to a major landmass. Or possibly somewhere in Southern Russia or maybe Turkey, Turkey might be appropriate given it's symbolism as a bridge between East and West.
You have to ask yourself the question though if there was a world government, would you really need a capital?
I mean if you're going to go the whole nine yards and completely eliminate Nationalism (and I can only pray iot will happen sooner rather than later), the very first objective of any world organization would be ascertaining the value and tradeability for a single world currency (which would create an uproar in the financial sector, particularly banking institutions, as currency trading is effectively halted).
You'd also have to decide what to do with national debts ... whether wipe them clean or create a new system of debt recollection given that populations would no longer be hampened by borders and political dead weight controlling trade and movement of goods and people throughout the world would be halted.
Then you have to create provisions by region to determine what civil services survive in this world ... universal healthcare and cheap education for all people of Earth and a redistribution of medical & educational staff and resources to every corner of the earth based on population.
Then you have to work out effective tax rates and the subsequent utilization of said tax in reconstruction, defence, R&D, education, medicine, transport, civil services (post offices, hospitals, police, fire brigade, libraries) ... etc etc
In the end such a capital would only really inevitably need would be a mint and an attached barracks filled with soldiers to ensure it's safety as the primary beacon of capitalism and exchange in this brave new world.