What’s the best treatment for agoraphobia?

Therapy, meds. Agoraphobia, or fear of crowds (literally of the marketplace) is amenable to treatment with psychotherapy and/or medications, including ssris and others. Discuss a referral with your pcp; depending on comfort level, they or a mental health provider can discuss medication options in addition to therapy.
Many different types. One of the most effective therapies is behavioral gradual exposure to crowds.