There is no single investment with the highest return. Different assets will perform differently at different times. Some investments, such as stocks or real estate, can grow in value over time. Other investments, such as bonds or CDs, provide fixed returns.
To get the highest returns on your investment, you must diversify your portfolio and invest in a mix of different asset types. If you're unsure how to do that, try investing smaller amounts of money in a diversified portfolio. This is where micro-investing comes into the picture - it is a great way to diversify your portfolio without investing a lot of money.