Fairy Liquid!!
NO WAY!!Why do so many of you use it?
Dishwashing liquid is for cowboys! Real window cleaners use professional detergents such as Unger's window cleaning fluid or GG4.
Why?
Fairy Liquid is... well, the composition is a bit like grit. It is great for getting grease of a plate, but not a window! You want a gentler fluid for windows.
Moreover, professional window cleaning detergents will make as much soapy bubbles, so if you miss a bit, less streaks.
But most importantly, your squeegee will meet less friction because there is not "grit"!!! It just glides.
Back when I first started window cleaner, I used dishwashing liquid for about a week. It was horrible stuff! Then I discovered Unger's fluid and haven't gone back to Tesco for my cleaning chemicals. I introduced a friend to it and he also has put dishwashing liquid behind him.

Time to raise the standard guys!!
Luke Jones FIEC
The Institute of Exterior Cleaning
A standard you can trust!