The best treatment for an allergic skin rash depends on the extent of the affected area. If only a small area is affected, using a fragrance-free moisturizing cream like Aveeno or Glaxal Base can help. For a more widespread reaction, Benadryl may work quickly but can cause drowsiness. Alternatively, Reactine is a good option as it only needs to be taken once daily and is less likely to cause drowsiness. It's important to consult a doctor if the reaction worsens or persists.