Speed Force supremacy — Diana is the better warrior, but Flash is too fast to hit.
Wonder Woman is arguably the greatest warrior in DC — better combat skill than Flash by a mile. But the Speed Force makes this matchup fundamentally unfair. Flash can think and react at speeds where Diana appears frozen in time.
Diana's bracelets and combat reflexes are superhuman, and she's tagged speedsters before. But a fully serious Flash operates at a tier where Diana's defenses simply can't keep up. Barry could vibrate through her sword, phase through her bracelets, and land thousands of punches before Diana processes the first one.
The Infinite Mass Punch is the equalizer — Flash can accelerate to near-lightspeed and deliver a single punch with the force of a dwarf star. Diana is durable enough to survive it, but not multiple hits at that force.
Speed Force makes Flash untouchable. Phasing bypasses Diana's defenses. The Infinite Mass Punch delivers astronomical force. Flash can think and act at speeds where Diana appears stationary.
Diana is the superior warrior by far. Her bracelets can deflect anything, her lasso can bind speedsters, and her combat experience spans millennia. She's tagged Flash before when he wasn't at full speed. Her divine durability can survive even the Infinite Mass Punch.
Flash wins through Speed Force superiority. Diana is the better fighter in every way except the one that matters most — she can't match his speed. If Barry is serious, the Speed Force makes this fight unfair at a physics level.
The Flash also fights
Wonder Woman also fights