Only tried a couple (AutoBrite Directs Magifoam, ABD's SuperFoam and ValetPro pH Neutral), I found the Superfoam required less product, but with the current VP pH Neutral I use about 1 inch in the bottom of a 1 litre bottle and that gives a nice thick foam that sticks quite well.
The initial outlay can seem steep depending on where you shop, but if over time you replace your pressure washer for a different brand, you can always get replacement connectors for the setup.
Fish