Nifty Index is 100% equity
NPS is a mix of Equity/Gilts/Bonds
MF depending on the type of fund could be balanced, equity or debt
PPF - the returns are similar to FDs where a fixed rate is given.
Since all of the above products are different, the returns would be different too. NPS is a pure retirement product and has added tax benefits - So in my view, the returns cannot be really compared.
However from my personal experience during the pandemic and when market crashed in 2020, the fall in my NPS was 0.50% whilst 20 to 25% of my capital was eroded in my direct investments in equities.
Only thing I want to add to the above post is that the stability, safety and EEE tax exempted nature of PPF returns makes it a noteworthy long term investment, especially when income is subjected to tax at the rate of 30% or more.
Between VPF and PPF, I prefer VPF as it gives higher interest ( same as EPF ). Please note, the returns on the investment beyond 2.5L/year on EPF+VPF will be taxable, whereas PPF returns are tax free