There isn’t a single “best” Fujifilm lens, as it heavily depends on your photographic needs and your budget. For overall versatility and great image quality, the XF 16-55mm f/2.8 LM WR II would be a great choice because it can capture a range of subjects.
If you have a Fujifilm medium-format camera, lenses like the Fujifilm Fujinon GF 30mm f/3.5 R WR and the Fujifilm Fujinon GF 20-35mm F4 R WR fit the bill for superb image definition and performance.