It depends on the use case. Always think about your end users. If your site is for people who will commonly be accessing it from a desktop, then go with desktop first. If it's for the general public or is fairly simple in terms of functionality, then go with mobile first.
It used to be really popular as a starting point to extend and customize. I'm not sure if this is the case anymore. All of the major ones (Material, Bootstrap, Tailwind, Ant) are good to use. There really isn't a "best" choice. It all comes down to preference for how it looks.