There isn't a "best" thread. My S16 has used metallics, polyester embroidery thread (RA, Floriani, and Isacord), poly quilting thread (SoFine), heavy cotton (King Tut), and any number of 40 weight cotton threads (Aurifil, Gutermann, others). The S16 will take them all, as long as you get comfortable adjusting the tension. Just pick the thread you like.
Pam