This question could not be answered easily, but I think I probably value my Ti dom as the most important.
I understand why others INTP's have said before that they prefer their 'N'. I would dislike seeing the world with the eyes of a dominant sensor. I really looove creativity, brainstorms, original solutions, perceiving "further" what is not evident. I agree at this point.
But now think in the concrete situation of an INTP: N<->S -> INTP<->ISTP. ISTP's have also intuition, in the form of Ni. This function is tertiary, not as strong as an aux but enough available. I know few ISTP's, but although not as high as me, they're quite intuitive, much more than the average S dom. They are introverted, they understand my way of thinking and have a enough similar personality. Being ISTP's aux sensors, they have enough intuition (tertiary).
But I would completely hate being a dominant sensor, if I were a Ne dom (ENTP) then I would probably answer than Ne is my preferred function.
Changing T<->F would make me an INFP. Altough my best friend is an INFP, and we understand and empathize each other easily, changing Ti with Fi is, in some way, changing our objective in life. And this is the point. My friend and me think in a similar way because we share the "idea generator" Ne aux. But as I said, we have a different "ego definition", Ti Vs Fi, our objectives are quite different. This is not the case of the ISTP's I know.
Changing I<->E would make me an ENTP, the most similar to an INTP without being an INTP. Obviously, if I had no choice, the preferred change. But only if the resulting ENTP is balanced. An unbalanced ENTP (weak Ti, Ne and Fe take the control) could be very irritant, as the majority of too strong extroverted for an introverted.
Changing P with J? I have no idea about how to value this, because as we share zero functions, I could not value properly a person whose mind is completely different. The INTJ behavior is in some points quite similar to INTP's (a third person could misunderstand a subject) but the core is literally matter Vs antimatter. How does antimatter taste? I don't know and never will know... (I could explode if I taste something :laughing

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Anyway, after these subjective valorations, the only objective answer is Ti. Because it is the INTP dom function, changing the dom function would produce necessarily the higher change in the subject personality (if we thake "preferred" as "the most important"). Oviously a person could like being a more different type, but because I like being an INTP, if I had to change something, I would like a change as less significative as possible. As function, Ti. The less important letter for me would be I because I would share the same functions and the same thinking core TiNe.