What preparation method yields the most caffeine? I want to make the most caffeinated coffee possible. What method should I use to yield the most caffeine content per liter?
The first thing you want to do is choose the right bean and the right roast.

Longer roasting removes some of the caffeine from the bean, so a lighter roast would be preferred. Additionally you should chose Robusta beans, since their inherent caffeine content is higher than that of arabica beans.

The next step is the grinding. The smaller you grind, the easier it is to extract "material" from the ground. This is due to the additional surface area you expose.

Then the last step is the extraction. The longer you expose the ground to water, the more substances you absorb. The extraction roughly has three steps:


Extraction of Acidity
Extraction of Sweetness
Extraction of Bitterness


Usually you try to stop extraction after the second step, to have the coffee taste nicely. By that point in time most of the caffeine is already extracted.

If you'd want to get all the caffeine out of a ground, you'd have to go through with extracting the bitterness, too. But for general purpose drinkable coffee it's definitely preferrable to stop after extracting the sweetness