YLI 100% GLAZED cotton thread - hands down!!! The importance is to get glazed cotton when hand quilting. It prevents knots.
I am currently hand quilting a quilt for a friend who bought the thread for me and it is always getting knotted up...I HATE IT!!!
I will say that i bought some Thread Heaven that i drag my thread thru which adds a waxy coating on it and that is helping, but it's just an extra step that I don't have when using the YLI.
I do have to order the YLI online because my local quilting stores don't carry it, but I dont care. I found a place that is very quick in turn around and i usually have thread in short time. YLI is also relatively cheap. a 400 yd spool is $4 and a 1000yd spool is $8