Mobile First or Desktop First?

We all know we need to change our corporate sites to be responsive. We simply want our sites to look perfect on all new devices including the smart watches. However, which one do we QC first? Do we build the mobile CSS first or the desktop CSS first?
The solution has now changed yet again. Today, it is now better to begin a custom theme and finalize the desktop design look and feel first. This is simply for the fact that we want to not only pass down all the data and design down to the smart watches but we also want to finish off with a new dedicated responsive stylesheet.
Historically, we used to use frameworks such as bootstrap, Omega, etc that will hold all the responsiveness. Now, it is considered superior practice to begin the entire theme from scratch.