What are some extremely risky mutual funds to invest for very high returns in 15-20 years?

So you are looking for the highest return funds and hence are willing to take on a higher risk! Since your time horizon is long, you can indeed take a high risk.

One can say that small or midcap funds take on higher risk but the returns cannot be guaranteed. Over a ten year horizon, many small cap funds have not given great return, though over last 5 years, they have given stellar returns.

The best strategy is to invest in a combination of asset classes as per your risk profile. In your case a mix of large cap, mid cap and multicap funds would be a good bet.

Do take a free risk profile assessment here and then decide how you would like to invest.