This all depends on what you’re comparing. First, if we talk about the type of coffee, then Robusta always has more caffeine than Arabica. Up to 25% more caffeine, in fact! This high caffeine content is also why Robusta is so much more bitter than Arabica.
Next, we need to consider the roast level. Some people think darker roasts have more caffeine while some argue that lighter roasts are stronger. Actually, both are wrong and generally, the caffeine concentration stays the same across different roast levels. What changes are density and weight, so a darker roast may end up having more beans and therefore more caffeine.
Lastly, we need to evaluate the brewing method and brewing time. A longer brew time will give you more caffeine but your cup will also be more bitter. A double shot of espresso will have more caffeine than a standard cup of brew coffee.