I am a fan of polyester embroidery thread for quilting...generally a standard 40wt. There are a zillion threads out there that work great for different purposes. If you haven't tried yet, a thinner thread (they range greatly, from 50-100wt) you might find you like for tight fills or any design that requires backtracking. My favorite threads are Glide by Fil-Tec for general quilting, Invisifil by Wonderfil for very dense heirloom style quilting, and Omni by Superior for a cotton look.