What is d most effective drug for vaginal itching?

That is a symptom: There could be any number of causes for the itching but the most common is a yeast infection. There are many effective treatments but you could start with a 3 day course of miconazole cream. If it worsens or does not respond you will need to be checked.

There isn't just one: It completely depends on what's causing the itch. Yeast (fungal) infections will cause vaginal itching and are usually treated with clotrimazole 1% cream which you can get in any drug store. Dryness if you were approaching menopause or have had your ovaries removed would need hormone replacement or perhaps a moisturizer. Sometimes even soap or other hygiene products are the cause.