I've experimented with thread (Uni 6/0) and different sizes of mono and fluorocarbon. I like the the Uni for dry fly fishing, and the mono for everything else.

For the mono, I have tried everything from Coats & Clark mono (0.005) all the way up to 10lb mono . In general, I have found that leaders made from the smaller diameter material, with more loops, perform better.

I found 4lb mono to be the heaviest that will turn out a nice leader. And leaders from 10lb mono... well if you're going after Moby Dick it migh be what you want, but not for me. They were horrible.