What is the best way to clean bathroom scum?

[#1]

You could try some Gumption Paste 750g Multipurpose Cleaner which is a mild abrasive cleaner. It's effective for polishing out marks on surfaces.

I've started using a diluted hydrochloric acid solution to remove scum in my toilet. It works quite well; you just want to ensure you don't leave it on so long that it eats into the glaze. You also need to ensure you are using adequate safety gear, including eyewear, gloves and a long sleeve shirt whilst using it. - MitchellMc

"Easy off BAM" and steel wool. Don't laugh until you've tried it. - Prof