I picked up an Audioquest Optilink 5 and it was remarkably better than whatever cheap feebie cable I was using at the time. I don't have to much to compare it to but an upgraded cable worked wonders for me.
I use a Sonicwave glass toslink between my ATV1 and Rega Apollo. Far better than an high priced fiber optic cable (fancy word for plastic) that I used. The Sonicwave cable cost me $25 with shipping included from Amazon.
I think Sonicwave makes plastic ones too. Make sure it's glass.