I use kevlar. You must guide it through an outlet like teflon tubing otherwise you will get chafing of the cable. I bought a reel of kevlar control line for about 8 bucks American. Should last for many a plane.

Tie the kevlar on an eyelet at your control end. Use a double knott with a drop of thin c/a and it will not come loose. Kevlar also eliminates the fear of interference. Cured a twitchy metal geared servo I had on rudder one time that I had piano wire on the first go around.

If your not sure of it try a test of it using a fishing scale. I pulled over 85lbs and could not break it.