100% cotton Guterman 50wt/3ply (sold at JAF) will work. The issue with that is that it sheds like crazy so you need to be constantly cleaning out the thread run & bobbin case/compartment. An 80/12 quilting needle will pair well with it on most machines. Some machines prefer a 90/14 needle with it.
I personally like Aurifil 40wt/2ply (sold at my LQS and also online at
http://www.hawthornethreads.com/). I use their 50wt/2ply also when I don't want the quilting to show up quite as much. Aurifil thread is absolutely beautiful thread that comes in over 250 colors. The 2ply means I can use a smaller needle (quilting or microtex 70/10) & it barely sheds at all; plus, it's colorfast with reasonable care. But it definitely isn't a "must have".