What line do you use? What brand(s)

Ever since braid was introduced I found reasons for using it. But as more companies started making, it I've found some brands cheaper and equal to that of the usual companies we've bought line from in the past.
Some use a fluorocarbon leader. Why?
Is mono dead?
What line test(s) do you use and with which lures?
Is line diameter significant?

A line recommended to me has a super-small diameter when going to 6# (2# diam.) or 8# 4# diam.) test - great for casting finesse plastics on light jigs! Line that casts light lures, is sensitive to light strikes at a distance and sets the hook with zero line stretch at that distance, is for me!

10 # or heavier braids also do well using larger heavier lures and for getting larger fish out of heavy cover.

Some winter thoughts. Opinions welcome.