Which material combination has the highest coefficient of kinetic friction?

[Surya97]

What has the highest coefficient of kinetic friction, steel on polyethylene, rubber on polyethylene, or polyethylene on polyethylene?

 

[Simon Bridge]

That would be infinity ... ie. complete classical adhesion.
The "coefficiant of friction" is just a model.

 

[Nidum]

The "coefficient of friction" is just a model.

Commonly known as the ' coefficient of fiction ' in engineering .

Actual friction forces as measured for real sliding surfaces are often far different from calculated values .

 

[Dynamics101]

What has the highest coefficient of kinetic friction, steel on polyethylene, rubber on polyethylene, or polyethylene on polyethylene?

I would say rubber on polyethylene.
I could be wrong though.
http://www.engineershandbook.com/Tables/frictioncoefficients.htm