Mouse is still the best. Wiimote is 2nd best. They key to good competitive FPS control is in the sensitivity of the pointing device.
And once again, by "best" I don't mean your personal preference. One reason UT3 multiplayer wasn't made cross-platform compatible between PS3 and PC is because gamepad users would get creamed.
The same would happen if a cross-console multiplayer FPS were released - the pointing device users would come out on top.
Actually that would be an awesome game, I'd call it "Console Wars"