There are many antioxidants. I don’t know how one decides one is better than the other, but I’d say in terms of simple side effects, melatonin is not exactly the most effective way to combat oxidative stress in the body. It may be a tool that can be used sparingly, but the hormonal disruptions it can cause (at least for me) are too great at anything but a tiny dose for me to put up with.
I’d stick with SOD, glutathione, garlic, vitamin C, and astragalus for a good base of combating oxidative stress and warding off bugs.
I take 15mg of melatonin at night. Tried astragalus about a year ago, two capsules as per label recommendation, got severe diarrhea. About three weeks later, tried again only one capsule this time, got severe diarrhea again so canned it. However, most of the herbs and supplements my late wife from the Indian sub-continent recommended to me have worked out well.
It is straightforward to rate antioxidants in terms of their ability to inhibit oxidation.