Medieval knights armor vs riot police clothing?

In what usage?

I believe riot gear is more lightweight and flexible than knight armor, but I doubt it would be able to resist heavy blows from many Medieval weapons.

It depends on what you mean by "better"

Modern riot gear is designed to withstand mostly rocks, bludgeoning, and light firearms. Most medieval weapons people know such as arming sWords, maces, and axes would probably be able to injure someone wearing this light body armor quite easily compared to the bricks and sticks it is designed to deflect.

Medieval knights went through several different armors over the years. Initially they wore heavy chainmail suits with thick padding underneath. These would stop most cuts and blows from many different weapons, but they were also cumbersome and expensive. They'd never stop a modern bullet.

Later periods saw the gradual adoption of metal plates in place of chainmail until the evolution of plate armor, which is what most people think of when they think of knights. This armor is heavy, but not cumbersome when worn. A French knight was famous for his ability to do cartwheels in full armor save for his helmet.

The steel plates have the same heavy padding underneath and really stop blows from things like hits, melee weapons, and thrown projectiles. But bullets could easily punch though the steel, which is why our soldiers and police do not wear it in the line of duty.

So the real question is, what are you doing that you need armor. If you expect sWords and axes, go for the knights' armor. But if you're looking at tossed bricks and occasional low caliber firearms, go for the riot gear.