As @Matt Grum says in his comment:
There's no correct sequence, neither is there a correct exposure, it all depends on what you're trying to achieve
For example if you want to achieve great DOF, then you want to lock down your aperture and play with ISO and shutter speed, now whether you change the shutter speed first vs ISO first depends on the situation, if you are shooting a still subject and you've a tripod (landscape for example), then you could lower the shutter speed. if you are shooting a portrait, then probably you'll try to play with the ISO first because people can't stand still for long time and if you have long shutter speed you'll get a movement.
Same goes if you're trying to get a specific shutter speed in mind, maybe freezing water drops or shoot long exposure shot of a waterfall for example.
Bottom line is, there's no priority, it's just about the effect that you want in mind, this will drive your settings to achieve the exposure you want