There is no single best medicine, as acne and rashes are different skin conditions that require distinct treatments. For acne, topical treatments like benzoyl peroxide, salicylic acid, or adapalene gel are highly effective. For mild, itchy rashes, an over-the-counter hydrocortisone cream or oral antihistamines can help. However, you should avoid applying steroid creams to acne, as this can worsen breakouts. Since a "rash" can have many causes (such as allergic, fungal, or contact dermatitis), it is highly recommended to consult a dermatologist for an accurate diagnosis and a safe, tailored treatment plan.