A false underhand grip results in the easiest pulldown because the best angle of pull for your biceps. The closer your hands are to each other. The more the biceps are involved.
Use either of the neutral-grip handles to put your arms in a stronger position. While you'll be able to use more weight, remember that your arms bear some of that extra weight. When you use the narrow grip, your chest also does some of the work. Keep in mind the extra muscle involvement, and don't make these neutral-grip movements the only back exercises you do.