What's the best approach for creating a secure authorative server?

Hi, I've managed to get my own UDP client and server implementation working with Unity. I've tried a couple of different approaches to make sure players won't be able to cheat.

So far I've come up with the following approaches:

1. Send keyboard & mouse input to the server, let the server calculate where a player should be and broadcast that data to all clients (including the original sender).

2. Send keyboard & mouse input to server and broadcast this to all clients, let each client handle movement logic based upon the input it receives from the server.

3. Calculate position on the client, send that data to the server and broadcast it to all clients except the original sender, because it already has the data.

To me it seems that my first approach is the most secure, but I'm not 100%, sure and maybe there are better ways or maybe I've overlooked something which is why I've created this thread.

Hopefully someone can point me in the right direction.

Thanks in advance.