I think the best folding bike depends significantly on your needs, as every good brand chooses a different set of trade-offs. Brompton defines the class; they are well-built, ride pretty nicely, and fold quickly into a small, portable package. They are also relatively heavy to carry, and they are expensive. Bike Friday has similar qualities, but to my eye they are less well-built and the folded package is larger. Many people prefer Bike Friday to Brompton, though, and costs are similar. Both of these bikes are a good compromise of folded size and ride.
The Montague is supposed to be a higher end bike, slower to fold and unfold, but offering a better ride for significantly more money. The Dahon's are less expensive and more widely available, but in my experience they are often heavier and less well-designed than Brompton. There are a few other unusual folders that focus more extremely on either ride quality at the expense of size, or size at the expense of ride quality.
Having had a Brompton, we now use some relatively inexpensive Chinese folding bikes. They are light and easy to get into the airplane. They don't ride that well (tiny wheels and a long handlebar shaft make them a little wobbly), but easily get us from the airport into towns. We don't use them nearly as often as I thought we would, as we just prefer to walk.