Green tea
It packs essential antioxidants like flavonoids and epigallocatechin gallate that can boost our immune system. In fact, both black and green teas are good. But, due to its fermentation process, black tea is stripped out of epigallocatechin gallate.
On the other hand, green teas also contain the amino acid called L-theanine, which can boost the protection of germ-fighting compounds.