It depends on your priorities. You have to start somewhere on the exposure triangle and prioritize accordingly depending on the shot you want. It depends on the creative impact of each parameter.

Examples from my photography:

• For landscapes, aperture is the priority to get the depth-of-field I need in the shot. Then I set the ISO as secondary to maximize image quality, maximize dynamic-range and minimize noise. I then let the shutter-speed be whatever is needed to expose the image as desired.
• For architecture, aperture is still the priority. I often set a long shutter-speed secondary to blur out people and then let the ISO be where it needs to be.
• For sports, I first set the ISO to the highest acceptable value for the intended print size. Then I set the aperture to something bright but maybe not the widest to account for focusing at the wrong distance. I let the shutter-speed set itself in that case.
• For creative work, aperture is the priority because it has the most impact on the image. If there is any movement, shutter-speed is next and finally ISO. If there is no movement, then ISO gets set before shutter-speed because, in such case, ISO has an impact on the image but shutter-speed does not.

ADDENDUM:

These are examples from my photography. None is an absolute rule either and I do select shutter-speed first on some rare occasions. The point is that each decision is guided by a mix of creativity and constraints. Creative options get chosen first and the last parameter generally falls into place. Even that, a carefully chosen ND filter can shift things to get something I deem a more desirable result.