Milk chocolate is ruined chocolate.

Edit: I'll elaborate. The primary health benefit of chocolate is the high antioxidant levels. So if a chocolate producer wants their product to lean on the health-promoting side, the last thing they want to do is include an ingredient that eliminates the primary benefit that a thing is known for. Dairy literally acts as an antinutrient in this sense, blocking the absorption of antioxidants in chocolate. It's been shown to do this to coffee, tea, and berries as well.

Soy milk would be a relatively better choice, but then again, like dairy, soy is one of the most common allergens. So I would argue oat milk would be the best choice for "milk" chocolate.