The most absorbent boosters are going to be the Bambino Quadro and their new Quadro Deluxe, which supposedly have double capacity.
The ABU PowerUps are very similar to the Bambino Quadro, as the capacity is very similar, however they were designed with different philosophies in mind: the Bambino Quadro is all about maximum capacity, and therefore has a low throughput. Conversely, the ABU PowerUps are designed with throughput (conductance) in mind.
If you're serious about boosters, I would recommend wearing two at once: a low absorption, fast wicking booster closer to your body, and a high absorption booster between the diaper and the wicking booster. An example of a good wicking booster is the Abena AbriLet Normal. The Maxi is kind of a middle ground.