There are assays for individual antioxidant enzymes for scavenging ROS. I did some works on them. you can read some of my papers. Also see original protocols in references section.

Dear Samarakoon, the best way to select the method is to know the particularities of your sample. What kind of compounds does it contain? Can any of the compounds present in your material interact negatively with the buffer used in the protocol? Or does one of the components in your sample emit the same wavelength as the buffer used in the protocol?

Yes, I agree.

It is better to use several methods with different mechanisms. Judjment based on a test is not recommended.

DPPH radical is a widely used method to evaluate the free radical scavenging ability of natural compounds. This assay is based on the measurement of the scavenging ability of antioxidant substances toward the stable radical so better to use this method.

Thank you all for adding views.

Thank you dear Olaniyi Amos.

DPPH, ABTS, DMPD, and many more methods are available

DPPH free radical scavenging is widely used one. It's easy to calculate IC50 value by DPPH method.

I would say that DPPH and ABTS are rapid, simple and inexpensive methods for preliminary determination of antioxidant capacities, therefore you could start with them.

It depends on the majority of the secondary metabolites in your sample. You could also share your sample by the polarity of the compound content using liquid-liquid partition and test them according to their polarity so that you could conclude which one will be the best for each polarity of your fractions.