which medication is best for anxiety disorder?

You're the medical student; you tell us.

"Xanax" is best anxiety Disorder.

... and so?

What studies verify this claim?

That sentence is a terrible train wreck of a sentence.

It varies greatly from person to person. Xanax is effective for relieving an acute episode like a panic attack, but is highly addictive so you can't use it too regularly. To relieve daily low-grade anxiety, something like an SSRI might be more effective. And efficacy and side effects tend to be highly variable among individuals.

Xanax (alprazolam) is a benzodiazepine. Alprazolam affects brain chemicals that can not be balanced by people with anxiety.

Xanax is used to treat anxiety disorders, panic disorders and anxiety caused by depression.