Nair is one of the most important moves for ZSS. It is save on shield, does damage, combos into many of her other moves, and sets up into stock taking scenarios. It also is disjointed, covers a lot of space.
Fair is a great combo extender. The second hit sends far, but can be controled to combo further. The first hit also combos very well.
Up air is great for jugglig opponents and pressuring platforms. It sends at a great angle, and leads to setups into up special.
A stall and fall, down air has niche uses in edgegaurding. It is very laggy.