There is no one best thread for FMQ. It all depends on the look you are trying to achieve. Don't want to see thread, just texture or do you want to ditch around applique and not want the thread to show? Go with a 100 wt silk or polyester. This thread is also great for when you have a very dense design and lots of backtracking as thread build up will not show.
Want the thread to stand out and the quilting to be "in your face" go with a 12 wt cotton (and yes there are 12 wts you can run through your machine, like Wonderfil's Spaghetti). Do you want your thread to shimmer and shine, go with a 40 wt trilobal poly. Want the thread to show but not be so obvious? Go with a 60 wt cotton or matte finish poly.
Need a touch of bling? A metallic or holographic thread will really make a difference.
Additionally there is no reason you can't use several different types of thread on the same quilt.
I highly recommend you see if you can get you hands on this book (either purchase or see if your library has it)
https://www.amazon.com/Guide-Machine.../dp/1574327968
Diane devotes an entire chapter to thread and how different types make the quilting look different with wonderful close up pictures showing the same FMQ design done in different weights and types of thread.
Sticking with just one type of thread for all your quilting is like just using one fabric for all your piecing.