I usually use a 50 wt thread...my favorites are King Tut (from Superior Threads) and Aurifil. But I just finished two twin-size quilts for young boys (very colorful) and used a variegated 30 wt poly from Superior Threads on the top and a 50 wt neutral for the back. Very nice to work with and I wanted the stitching to show up a little more on these quilts. Basically...whatever you like.