While all of those might be true, you can do something better by channeling a possession spirit who then endows you with the Astral Form ability, which means you (including your body) are in the Astral Plane, without the normal time limit for being on that plane. And all of what I just said can be done by a mystic adept with a possession tradition. That's why I ask why you would spend the extra 5 BP to become a magician when you can also do this by being a mystic adept.
It also requires you to be of a possession tradition, which then limits your options for summoning.
A: What are the best spirits? I haven't heard of any must have spirits.
Of the core, I like Man. I'm a big fan of Task, Combat, and Ancestors, as well. Gives a nice range of powers, including some Spell options.