"Plans are useless; planning is indispensable," according to Dwight D. Eisenhower, then Commander of the Allied Forces in Europe during WWII. Now, you may be in total agreement with the first part of that statement, but you are really not convinced of the truth of the second part.
However, the best business plan template is that which meets all your business needs and requirements by offering you a high level of customization and a professional layout of all the content that you would like to include.
But your idea may be so new that you may also need to talk to potential customers, host some focus groups, talk to an ad agency, or maybe even make a prototype and float it past some people. Be prepared to spend the time. Remember, it's not about the Plan but the Planning.
Are there emerging markets or market segments? Where does this product or service currently perform best? Possible Data Sources: trade associations; trade journals; attorneys & accountants dealing with the industry; industry salespeople; state business websites; focus groups.