Which cleaning agent do window cleaners use? I ...

A water purification system that uses reverse osmosis. For details as to how this system works, see this web site. As far as buying it on the Internet, you need to hire the company.

water and vinagar or water and rubbing alcahal.you can go to autozone  and they have a product called 20/20 glass cleaner,it works great

I don't know if there is a best cleaner. I like generic windex.

Use a squeegee

I love Glass Plus....no streaks..perfecto!!!

It depends...

There are 2 main methods to cleaning windows:

1. Old fashion way of using cleaning solution (vinegar, ammonia, windex, etc.) and something to wipe it off with (paper towel, newspaper, microfiber cloth, etc.)
2. The professional way of a using a squeegee.

Either way, I think that the cleaning agent is not as important as how you remove it from the glass.

If you decide to go with method #1 (cleaning agent & rag) use a cleaning agent that evaporates quickly (like windex) but make sure to buy a microfiber cloth to wipe the window dry.  It is the only way to go.

However, if you want your windows to be perfect, you definitely need to go with method #2 and learn how to use a squeegee.  In which case, the cleaning agent can be as simple as a gallon of water with a little bit of dish washing soap.

Or, you could always hire a professional window cleaning company .

Don't wet your pants but window cleaners get a bucket of water with a couple of drops of Dawn dishwashing liquid and their trusty sponge and squeegie.  Sorry but I just told you the truth. Try some newspaper for final cleanup if you have an OCD for clean windows.