Our Head YOUTEK Extreme MP 2.0 Poly demos are strung at 53 lbs with Head Sonic Pro 17 (4 lbs less tension than our multi demos).
We carry quite a variety of soft poly and co-poly strings, many of which have reviews from customers as well as TW playtesters. It would be very difficult to choose a top 1--or even top 10--but here are a few to start with:
Volkl Cyclone 17
Tecnifibre Pro Red Code 17
Polyfibre TCS 17 (1.20)
Kirschbaum Pro Line II 17 (1.25)
Hope that helps,