The answer to that will completely depend on your risk appetite. PPF is a 100% Govt backed scheme with almost ZERO Risk and Fixed Returns of 7.1%. ELSS and Mutual Funds on the other hand have higher risk, and offers variable returns of 10-13% over long term.
Since you are already investing in a long-term investment scheme like NPS. Therefore, investing in PPF won't make much sense.
Besides, I think your decision should be based on two facts:
1. Your risk appetite
2. Your goals
If you have short-term goals and you can take little or moderate risks, then there are better options than PPF. Like you said ELSS funds, Mutual funds etc.