Usages of "challenge"

Hi,

I would like to know how to use the word "challenge".
I got some examples of sentences.
I think all 6 usages could be correct, but want to hear some opinios.

Sentence1:
One of the challeges facing our clients is the high production cost.
Sentence2:
One of the challeges for our clients is the high production cost.
Sentence3:
One of the challeges of our clients is the high production cost.
Sentence4:
One of the challeges facing our clients is to reduce the the production cost.
Sentence5:
One of the challeges for our clients is to reduce the the production cost.
Sentence6:
One of the challeges of our clients is to reduce the the production cost.

Thanks