I use this http://rain-x.kcci.eu/rain-x-products/rain-x-rain-repellent/ and it seems to do a fairly good job.
Rainex is only ok if you don't use wipers, and it does work above about 30mph and the beaded water blows up the screen and away without wipers, but below about 30 you still need to use wipers / screenwash and the rainex effect soon starts to wear off and I found it then causes smearing, so stopped using it. Ok on side Windows but I will never use it on windscreen again. There was also a washer bottle screenwash that I tried that had rainex in it, when I found out how smeary it was I had to empty the reservoir and refill with ordinary screenwash but it took a long time to get screen back to normal clear vision. I think that because rainex is supposed to cause the water to form beads instead of spread out the washer bottle fluid (surfactant / detergent) are fighting each other - one wants to bead and the other destroys the surface tension of the water and wants to make it spread out, so the compromise is that the water forms smaller and smaller beads which actually are not cleared properly by wipers and form like a smeary mist on the glass, which is horrible at night.
I will use turtle wax clear-vue window cleaner now and if squashed insects to remove I spray it on screen and use a kitchen scourer ( the sponge with the white nylon antiscratch fibres on it, do not use green or brown as they will scratch glass ) to wipe screen over before using some more cleaner and kitchen roll / a cotton cloth to polish.