Actually, I think it would also have to be the fastest.
If you decelerate, you will fall out of orbit, not rise.
If you decelerate while trying to maintain the same orbital radius, you will fall to a new, lower orbital radius. However, that was not the question set. If you wish to gain a higher orbit in the same orbital plane you presently occupy, most efficiently, you must lose forward velocity as you gain altitude. You must shoot the jets to declerate your forward momentum, as you raise your altitude, and, if I'm interpreting the math correctly, you must lose velocity proportional to the square of the gain in radius. In rough language, that means you have to fire the jets a lot forward, and a little down, for the most efficient burn to raise orbit. Think of it this way: you are being flung forward, away from the earth by centrifugal force, and orbiting, instead of buzzing off into the great beyond, because Fgrav and Fcentri are exactly balanced. But Fgrav varies as the square of the distance, and Fcentri varies as the distance. If you fire directly forwards, you are firing tangentially against both vectors, but you are in orbit, which means your jets are constantly turning "downward", toward the earth, and, Fgrav weakens faster than Fcentri so if there is any least downward component of your accleration, and I aver there is, you will go up, at least in the beginning, if you fire directly forward.