GBE are constantly merging so there isn't really a single server.
I could say:
EU - SUNA
NA - SEANERA
SA - HONBRA/OMBRA
But its subjective.
I think these few servers I listed have the biggest whales/rare collections, but maybe I am wrong.
They're all equally small in population.
This is why the discord exists, the community is only big enough when grouped together.