Prism99

For precise piecing, my favorite by far is Aurifil 50wt. From what I have read, Presencia 60wt is even better.

I prefer polyester thread for quilting, but cotton thread for piecing. This is because cotton thread is "grabbier" than polyester. This means that when chain piecing with cotton, the ends stay put even if I cut the threads quite short. When I pieced with polyester, the threads had a tendency to slip out of the last couple of stitches when chain piecing, even when I cut the threads longer.

Be aware that weight is not everything when it comes to thread. I used to piece with Mettler 60wt cotton thread; however, once I tried Aurifil 50wt I never went back. The Aurifil thread is much smoother and produces way less lint in my machine. I think the smoothness of the thread makes my seams more accurate, but I'm just guessing on that.

Presencia is supposed to be the very best for piecing, but has always been a little pricey for my blood. One advantage of Presencia is that, even though it is a 60wt thread, it is 3-ply (which makes it stronger). Aurifil 50wt is 2-ply (which makes it weaker even though theoretically it is heavier). However, I have never had Aurifil break on me while piecing, or in a seam, so I stick with it.

I was surprised at how much more accurate my piecing became when I switched to Aurifil even though I did nothing else different. I think this is because it presses into the seams, so well it takes up very little space in the turn-of-the-cloth.

Edit: I don't think it helps to use 80wt in the bobbin if the thread is polyester. Again, this is because polyester thread is so much more "slippy" than cotton thread. And it would need to be poly at that weight to be strong enough for seaming. I did try a fine bobbin thread for piecing (the kind that is sold for embroidery) thinking it might help with accuracy, but did not like its slipperiness. After all my experimentation, I found Aurifil in top and bobbin to work best for me.