I've used Dr. Clay's solution, windex, alcohol, and acetone. The best is Purosol.

The problem with many of these solutions is that (a) they have water in them, which tends to streak; or (b) they are hydrophilic, meaning, like acetone, they suck water right out of the air and then leave a streak on the glass.

Purosol link

I buy them two at a time to reduce shipping cost per bottle, a bottle lasts me over a year. You can get additional information on the astro-physics web site which sells Purosol as part of its kit. Purosol works so well I've never needed the rest of the kit.

regards
Greg N