What is the best way to have your site automatically use HTML site for PC and Laptop, and a "mobile" site for all mobile phones automatically?

This really needs to be determined by you. Only the designer knows what will be easier and faster to design and still work with their specific content. I'm using a responsive site for the two sites I'm currently building. I have a few sites that use redirects for mobile users because the older existing content needs to be converted to responsive. I like the responsive direction because you only need to design and maintain about half as much data.