Of all the soft co-poly's I've tried, VOLKL V-Square was the most impressive for spin. It is crazy soft and stretches as you string it, holds tension much better than cyclone, works at a wide range of tensions, and lasts OK, especially at it's price point. Lower powered and stiffer when strung at higher tensions like 53+.
https://tt.tennis-warehouse.com/index.php?threads/best-soft-string-for-spin.260228