Danville's Fly Master(+) 140 & 120 Denier are good for medium and large flies respectively.  140 D for sizes #8 up to #4, 210 D for #2 to 5/0.  100 yard spools 

Danvilles Flat Waxed Nylon (210 denier) is flat, the nylon fibers are exactly parallel like shiny floss with very bright colors, 100 yd spools.

Super Fly and Perfect Hatch are 100m, and 100 yd respectively.  Their 3/0, 6/0 and 8/0 sizes are all much thinner other threads and are very strong for their diameters.. 

Veevus GSP Gel Spun Polyester is a super thread, super strong and about unbreakable is all sizes.  75 meter spools are the best deal anywhere. 

Uni 6/0 (136) denier and the thinner 8/0 (72) denier are great threads for medium sized flies and small trout flies respectively.  Economical 200 yd spools.