Typing, playing guitar and playing piano - quickly - are all examples of 'muscle memory' and they require progressively less brain activity to perform, so it's not really a matter of quickest brain-to-finger activity.
That said: Typing requires the least physical movement of all three, so typing would be fastest.
Between guitar and piano it depends to some extent on the music: Rapid repetitions of the same note would be faster on a guitar (either an up- and down-stroke of the pick, or thumb and three fingers all striking the same note in rapid succession), whereas - for example - a repeated figure involving a series of notes which fall conveniently for a pianist would probably be faster on piano. A repeated pair of chords would probably be faster to perform on piano too.
But generally, piano requires more force to sound the notes, so it's a slower activity than playing the guitar.