Q: what is the best cleaning solution for the imac screen?

plain water using the cloth that came with the computer.

The one recommended in the user manual that came w/your iMac = water.

Check out > How to clean Apple products

l use a large microfiber cloth dampened with warm water. Wring it out well so there is no excess water left in the cloth. I fold it up and use either vertical,  single strokes going from the bottom of the screen wiping toward the top.

OR I use single, horizontal strokes from left to right starting at the bottom of the screen and moving up successive, single,horizontal strokes until I reach the top of the screen.

Does a real good job on my screen and it minimizes streaking.

you need to let him know if you do this with the imac on or off ?

iMac is shutdown.