For piecing, my absolute favorite is Aurifil 50wt 2-ply (top and bobbin). However, I also like Threadart's 60wt polyester bobbin thread for piecing (both top and bobbin) and it's a lot less expensive. I have tested this poly with an iron to make sure it doesn't melt, and it has not melted for me. Here's a link to it:
http://www.amazon.com/Bobbin-Thread-...dp/B0034W118Q/
With both of these threads my seams lie flatter and my piecing is therefore more accurate.