My advice would be to start with the opportunity first unless you have a founding team that has been wildly successful before. In that case start with the team as that is ultimately what they are investing in anyway.
The investors are looking for the problem and the ultimate size of the market that might use your product to solve the problem.
Take a look at the following link, it shows actual pitch decks with comments from 9 well known funded startups: Page on onboardly.com
Edit to add link to new relevant article: This is how 500 Startups, Sequoia and others want you to pitch
Source: Quora