Jodan Zuki.

[SamsIAmz]

I originally practiced horiztonal for several years. Then more recently I have started doing 45 degrees. I find it a more natural feel, but you won't have the snap of a horizontal punch unless you have practiced horizontal for a substantial amount of time. I still go back and practice horiztonal sometimes.

[jaypo]

I've read that the best punch scientifically is in the middle! Hand slightly tilted. Supposedly, this allows the proper alignment of the bones in the arm and hand to produce the most focus and power.

[sensei8]

It's about matching weapons to targets. As shinobi points out, distance is one factor that effects this. So will angles between you and the target, the dynamics of your movement and his. Relative heights and stances. All sorts of things.

Whatever fits best that will allow me to deliver damage.

Solid post!!

A shorter person will have to augment their fist to contact the forehead squarely...for example...Refer to time stamp 1:05 of the following link...

and 4:00 of this following link...

[ossemon]

Striking seiken or tateken is all depending on several factors, many already named.

However: Striking with only one option isn't very kata, but rather fairly dead in options. Being able to strike quickly the most effective way is the goal, not the fist orientation itselft.

Striking from below and up close usually calls for a tateken zuki, but it varies from style to style.

[JohnnyB]

Horizontal 100% of the time BUT when doing Kizami Juki after a Block like Uchi Uke or Soto Uke then my hand sacrifices technique in favor of speed and I see my fists punching vertically or 'in the middle' as jaypo cleverly mentioned.