H2Go Rain Repellent from Angel Wax. As long as you prep the windscreen well (clay and clean first), this stuff beads at under 30mph. Very small drops - really great.
Tried many of the popular options in the past but think this is the best thus far - any by far the cheapest decent one. For me, beats the Gtechniq range - my prior favourite.
(screen capture from my latest toy - a Dashcam .. report to follow once installed professionally!)