What is the most important thing to Margot from "All Summer in a Day"?

Sun is the most important thing to Margot.

For Margot, there is nothing more important than the sun.  She is from Earth, so she remembers the sun.  Not only does she remember it, but she cares more about the sun than anything else.

The other kids in Margot’s class have never really seen the sun in their lifetime.  It last came out seven years before.  Margot, on the other hand, remembers the sun.  That memory dominates her existence, and makes her different from the other kids.  They are jealous of the fact that she came from Earth, but coming from Earth makes the constant rain even harder on her.

There was talk that her father and mother were taking her back to Earth next year; it seemed vital to her that they do so, though it would mean the loss of thousands of dollars to her family.

Margot keeps herself apart from the other kids.  She doesn’t really understand the other kids, and they do not really understand her.  She is obsessed with the sun.  It is all that matters to them.

When the class sang songs about happiness and life and games her lips barely moved. Only when they sang about the sun and the summer did her lips move as she watched the drenched windows.

She does not ever get involved in the games the other kids are playing.  She won’t play with them and she barely talks to them.  When she does interact with the other kids, they are rude to her and bully her.

Unfortunately, the other kids in the class do act very cruelly to Margot.  They lock her in a closet so that she misses the sun.  This is something that will utterly destroy her, because she needed the sun even more than the other kids.