Superman wins in a fight. They both move at the same speed. As fast as they want. The only difference is Superman has infinite strength, insane endurance, durability, heat vision, ice breath, x-ray vision, solar flare, etc.... the list goes on, and on, and on.

Flash is fast. That's about it. He can't throw an "infinite mass punch" as he would literally become a singularity pulling the entire universe into himself in fractions of a nano second.