If I wrote this post a long time ago, before MPs and prime ministers existed, the answer to that question would be very easy, the Queen (/King) of course! The Queen/King used to have all the power, they would make the law, they would deside everything that happens in our country. But now they are used for a more ceremonial purpose. The members of parliament make a law or something, and then the Queen/King approves of it, and makes it a thing. But they could make up the laws if they wanted to.
I personally think they have the same amount of power, but what do you think?
Thanks for reading :)