Mathematically, they are all equally important. This is one reason why water cooling as efficiently as possible is a balancing act and can be difficult.
If you mean what will cool better, a straight fitting flat on the die or a maze of fins and crap, that would depend on flow rate and pressure. Trying to make two blocks with diametrically opposed methodologies would simply prove that they are equal according to Fourier's equation.
I will add that the easiest thing to do to help yourself out is to use copper get the highest (practical) thermal conductivity for your block. In my mind, that's a given.
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