For me the number one has to be a swivel neck! This is the one true thing that I couldn't work without. It just makes life SO much easier and more efficient. Any window at any angle, clean glass right down to the floor without bending down, or really even moving your body. Clean sills and sash runners with ease- and with a normal brush. Clean velux without changing the angle of the neck. I honestly don't know how people manage without it. Everyone that I have personally introduced them to have had the same reaction- "wow, how did I ever manage without it!"
Secondary to that would be the Univalve.
Next would be Gardiners 1.4mm jets. I'm now using around 40% less water with no negative effects! Pump speed has gone from 30-35 down to 18-20. Less water, increased resin efficiency and the next time I change my van I'm downsizing to a berlingo size vehicle. I can do a big, full-on day with 300/350ltrs now easily!!😎👌