To me, priority A is the best choice for magic, not only giving you a magic rating of 6, but also 10 spells and 10 bonus points for skills. Skills and attributes are less important - skills because while you can easily go priority B for them, you don't really need to if you focus on only the most essential of the magical skills, attributes because your main two attributes to boost are willpower and your other drain stat. Sure, you want an average body, decent reaction, and decent intuition, but mages still wind up with at least one or two attributes that can be relegated to dump stats. Resources are only important if you decide to go with magical foci. Metatype is good for Edge, since Edge can not only give you extra dice, but uncap hits (especially important for direct combat spells).
I tend to prefer fiddling around with combat mage builds, so my favored priorities would be A magic, B attributes, C metatype (human, for a 7 edge), D resources (force: 4 combat spell focus), and E skills (covers essential magic skills at a high rating, and lower ratings for "every runner" skills). I know that the last one, skills, is not enough for a lot of people, even though it actually works out to 28 skill points plus any karma purchases - some players have more expansive views on what is "essential".