This question is difficult to answer for a couple of reasons.
First, it's not really possible to compare the strength of coffee in absolute terms. There are many different types of coffee beans with varying levels of strength, and there are various ways to prepare the drink which also have varying levels of intensity. It might be helpful to think about this question in relative terms instead.
Second, studies have found that people who regularly drink coffee vary in their responses when it comes to how "strong" they prefer their coffee. Some people enjoy strong coffees with strong flavors while others actually prefer weaker versions that favor milk or sugar content over things like bitterness or acidity that come with darker types of espresso-type drinks what some people see as stronger.
Picking the person's favorite roast may or may not make them happy, even if it "might be" stronger. So I'll assume you want to go with strength here. Obviously, Espresso tops out on this scale of strength, but it has a very distinct flavor profile that some people enjoy less of than others. Darker roasts are weaker than lighter ones because their flavors are masked by the dense dark roast creating a subtlety between your taste buds and the actual coffee beans flavour flavours- which means they need more roasted coffee beans to create that same level of intensity in each cup.