Lately I've been using Masterpiece by Superior Threads. Connecting Threads has a bobbin donut with 36 different colors on cardboard bobbins. They also sell YLI silk thread in a neutral set. Keepsake has it in more colors. Aurifil 50wt works well also. I used to use Mettler (silk finish? I think) which was also fine. I confess to using doubled thread for convenience. But my official advice is to use one strand.