Depending on context, all of them or none of them.
Bitcoin could have used any genesis block. The genesis block is not a normal block, and is not subject to the normal block validation rules. It is definitionally the 0th block of the Bitcoin chain, and thus unconditionally valid. Its contents is entirely irrelevant, and so it fields could could anything.
Bitcoin uses a specific genesis block and no other. However, the genesis block is definitionally the 0th block of the Bitcojn chain, and no other block is. Changing anything in it, and it no longer meets that definition. So nothing can be changed in it.
