I'm guessing you're talking about keyword selection for SEO. IUsually emerging technologies, industries, products, services are the best keywords with the lowest competition. For example, if you find an industry or product no one has ever heard of. You can create a website around this. You become the authority on the product or industry. As the industry grows so does your traffic. There is a lot of guess work involved, you don't want to put your eggs all into one basket, and its hard to tell the future.
You're question didn't really address products that people are searching for, but I've found this to be a very sound strategy. If you're going for longterm and you have the flexibility, this is my recommendation.
Do you have a specific product that you're looking to promote? Are you trying to select a keyword for SEO or SEM?