Quickening should always be first unless you are going for a specific build that requires something else (like Channeling).
Masking can help with a lot of stuff and is generally useful for any runner to not be noticable as a magician so easily.
Then, if you're focused on spellcasting, I'd suggest Flexible Signature. You can a lot of spells, don't want to be tracked super easily. However, if you don't think it'll come up either way, then skip this. You could maybe go for Spell Shaping if you sling a lot of AoEs.
For spirits, I'd suggest Channeling (if you're a possession tradition, definitely, if not, just if it's your style) or Summon Great Form Spirit.
By your fourth one, Centering (with a Centering Focus, which you should be able to get by now) will make a sizeable difference.
Fifth and beyond, I'd say it's up to the player. Shielding is a good choice for pure defensive purposes, but if you're going for something specific, do that instead. Shielding can wait.
If you're alchemy focused, this list is totally different, so nevermind. Same for various Aspected Mages.