One of the few things I miss about the US is Death Wish Coffee. Their trick is that they're a high-end, dark roast robusta coffee - most high-end coffees are arabica, which are more refined in general but lower caffeine.

There are now coffee beans (by other companies) which are infused with caffeine. I think it's cheating.

(Also, if you took the Death Wish beans and light-roasted them, you'd get more caffeine. They would probably not taste very good, though.)

Death Wish now lays claim to being the dark roast with the highest natural levels of caffeine. But for someone like me, who loves dark roasts and caffeine, it occupies a special place in my heart and now in my memory.