I've been using Gordon Griffith 14/0.
While I don't use that supplier anymore, I do love using the fine thread and the ultra smooth fly's I can tye with it.
Fine threads can be a challenge if one tends to be heavy handed with their tying. But with a rotary vice they are really great for building bodies and heads.
Instead of wrapping with it, you let the rotary draw it from your bobbin as you guide it into place.
I like that for building body shapes and heads.