Usually vanilla CSS (as SCSS). When I have the opportunity to work on projects from the beginning, I always investigate (and sometimes implement) SASS for primarily 1 reason – variables. If early in a project and still nailing down colors & spacing, it proves to be a significant time saver. It's easy enough to make a break from a simple SASS build and deliver files as CSS and give (the next developer) a chance to use SASS later - should they choose.