my go-to is Sofine #50 wt. i also like Omni & Permacore, will use Glide if I want shiney. I also have some Marathon #60 Trilobal in a gold and a silver that looks like metallic without being metallic. I purchased 3 tubs of thread cones when I bought my Gammill used 4 years ago--so have lots of thread. my machine doesn't like Sulky (it shreds unless I go slow) or YLI cotton (same issue). King Tut is fine, but it seems so heavy compared to a #50 or #60 so have to keep that in mind. I rarely use prewound bobbins, but will if I'm using Bottomline or the #60 Trilobals on top cause I sometimes get the #60 and finer threads wound too tight--so then I use either Superiors' Bobs or Magnaglides.
I recently bought several neutral colors of Superiors MicroQuilter for fill work and loved it.