I personally like Montserrat the best of the given options. Open Sans is bit too basic, I don’t like the kerning on Muli, and Poppins just feels off; it’s too clunky.
Bear in mind that’s just my opinion, and to be honest the differences between Open Sans, Muli and Montserrat is quite small.
What prompted you to pick these fonts? And why not go with a default font such as Arial, etc, that’s not as heavy to load?