Is it the best choice to top up my CPF for the compound interest starting from now or is waiting out for other benefits (if any) the better choice?

Personally, I would not top-up the CPF SA at aged 21 as you still have over 30+ years to go before you can use the money! I would say to keep your money liquid in the time before you reach the withdrawal age for short term use.

Yes, the compounding effect can turn you into a millionaire on paper, but the funds can only be accessed after you're 55. Think about your own goals and needs.

What would you be doing in the time before you can withdraw that money? What would you do with that large sum of money at age 55? ​​​

There is no harm starting early to take advantage of the 4% interest rate that will have compounding effects in the Special Account. Just my humble opinion.

Better choice? No at your current age.
Putting in cpf = can only be withdrawn at age 55

I am confident in getting better returns with investment

Exploring your choices

If you are very confident you can get higher returns from your investment, then by all means go ahead and invest. This is because CPF can only give you 2.5% or 4% and you can only withdraw it when you are 55 years old.