The correct answer is “If”.
The word “if” is used to introduce a condition or supposition. In this case, the speaker is imagining a condition in which they are the richest person in the world.
The word “unless” is used to introduce a condition that must be met in order for something to happen. In this case, the speaker is not imagining a condition that must be met in order for them to be the richest person in the world.
The word “when” is used to introduce a time when something will happen. In this case, the speaker is not imagining a time when they will be the richest person in the world.
The word “not any” is used to deny the existence of something. In this case, the speaker is not denying the existence of a condition in which they are the richest person in the world.