Gardiners Superlite brush. Best brush in my opinion in terms of lightness, scrubbing power and rinsing on. Brush stays very clean, even after bad first cleans. That's the non flocked version, the flocked version might be a bit better in cleaning power, but you can't rinse on as well as with the non flocked brush.
The only other brush that could beat in in terms of scrubbing power is the Tucker brush, but that one weighs much more, and you need to convert it to a normal screw plate. Also costs roundabout £60 including postage.