It depends on what you mean by "hardest". If you're talking about the most competitive, it's probably anything related to finance, insurance, or healthcare. These industries are heavily regulated, and there are a lot of established players with deep pockets. But, if you're talking about the most difficult to rank for, it's probably anything related to technology, software, or e-commerce. These industries are constantly evolving, and there's always new competition popping up. Ultimately, the best niche for you is the one that you're passionate about and that you have some expertise in. If you're willing to put in the work, you can succeed in any niche.