This one will be easy really.
Wiimote : 9/10
1 point lost, because though it's by far the easiest to get in seconds, the controls can be confusing when you're new to these kind of games. But everybody can grasp the concept right away, and begin playing even if they never touched a computer in their life.
Gamepad : 6/10
Analog sticks are complicated to grasp. You have two of them to master, with your thumbs. Those are completely non-natural moves, that you take time to learn. It works so bad, the games have to put things like auto aiming for it to be acceptable. With increase in games display resolution, it can only be worse.
Mouse : 1/10
Completely unnatural, scare most people right away, and you need a long experience with moving a mouse (takes at least a week), clicking right or left, and you have to use the keyboard, which is very user unfriendly for games. That's the worst control of all. No wonder people gloat about mastering these, as it's so complicated.
People that want to have fun, and not learn lots of complicated things before having fun, will go Wiimote right away : best control ever for FPS. Must be why arcade cabinets have light guns and the like, and not mouse or gamepads...
It's also more natural, not like FPS with the mouse, where the viewer can do amazing things, like strafing as fast as he runs forward, make U turn while running (which should mean they can make saltos and all, but never do) against a wall without bumping the gun against the wall, regain health after some seconds or after walking on items, and things like that. What's funny, is that hardcore FPS players talk about physics in these games, despite all these nonsense.
