What is the best anti-anxiety drug?

It depends. Anxiety comes in different types. Panic attacks, bad worrying, phobic fears etc. In fact anxiety comes with almost all psychiatric disorders in adults. Think of it like a symptom and talk with your doctor to get a diagnosis and then the right drug.
Best. There is no best, all work similar, differ in length of time they work for, side effects, and tolerability.
Good Question. There is not an automatic answer to this. It really depends on the person's situation and what is needed from the medication. The most potent anti-anxiety medicines that are used much these days are the benzodiazepines such as ativan, xanax, (alprazolam) and klonopin. These meds are helpful but can be addictive over the long term. Several of the antidepressants are useful for anxiety, but work more gradually.