I vote for Aurfil as my favorite 'high-end' thread for machine piecing. it stays in place and doesn't unravel at the ends by the time you go to join into blocks and rows. And I quilt with it too - so pretty!
But I have learned to use So-Fine and Bottom Line without the unraveling and - other than being hard to pick out stitches when I have to undo something
it is a nice flat seam.
Don't tell the QP I am using poly on cotton pieces LOL