Which one is more acidic, H3PO4 or H2SO4?

Based on pKa, sulfuric acid is the strongest acid.

Sulfuric acid has two pKa values, one, each for the following equilibria:

H2SO4 = HSO4(1-) + H+ pKa = -3.00
HSO4(1-) = SO4(2-) + H+ pKa =1.92

Phosphoric acid has three pKa values, one, each for the following equilibria:

H3PO4 = H2PO4(1-) + H+ pKa = 2.16
H2PO4(1-) = HPO4(2-) + H+ pKa = 7.21
HPO4(2-) = PO4(3-) + H+ pKa = 12.32

The smaller the pKa value the stronger the acid. All of the pKa values for sulfuric acid are less than those of phosphoric acid, hence sulfuric acid is the strongest acid.

Sulfuric acid is more stronger than phosphoric acid.sulfur and phosphorus is present in the same period , sulfur is more electronegative and easily hold its electron so on dissociation they form HSO4 negative ion, which is stable as compared to H2PO4 ion.