If your metric is the length of the land that the map boundary cuts, then the best centre lies somewhere in the Indian Ocean. Proven here: [1]
[1] https://marcinciura.wordpress.com/2015/11/17/slicing-earth-c...
[1] https://marcinciura.wordpress.com/2015/11/17/slicing-earth-c...
edit: I didn't even know about this: https://en.wikipedia.org/wiki/Land_and_water_hemispheres