By best way, I mean easy and efficient.
Mix a dishwasher tablet with water in a large spray bottle
Spray grout and let sit overnight. Wipe clean in the morning.
it's like magic
Bleach is my go to and a scrubbing brush. There are natural way but they are hard work, and who want to sit scrubbing a shower all day.
Try this: https://www.hometalk.com/22765530/neat-swifter-hack-4-ingredient-diy-bathroom-tile-grout-cleaner
My two favorite ways to clean grout is a baking soda and peroxide paste and use a toothbrush to clean the group. Another idea is having a steam cleaning company to steam the group.