To create a great mission statement, the best resource is missionstatements.com which gives tons of examples.
However, I don't think that's going to solve your challenge. Writing a great mission statement has little to do with pitching investors.
Rather, investors want to understand why your company is uniquely qualified to succeed. They want to understand the pain you solve, how you solve it, and the financial implications of your success. These are the building blocks of your business plan and executive summary. You need to understand these elements first, before reaching out to investors. Once you have a firm grasp of these components, you’ll be better positioned to explain your company’s role in the marketplace and how that will shape your mission statement.