If you would ask what Giants or Fortune 500 companies are using for diagrams and flowcharts, the answer would be LucidChart.

LucidChart is one of the premium application used by the brands like Coca-cola, Dropbox, Mozzila, and Tata. It comes with a simplified dashboard and secure cloud management. 

The single user version of Lucidchart is free which is good for solopreneurs and freelancers. Big businesses and startups can use enterprise version which has more features like the third party integration and team management.