I have as much thread as I do fabric
1. Aurifil
2. Superior
3 Isacard
4. Robison-Anton
5. Wonderfil
6. Glide
7. Star
8. C&C
9. Connecting Threads - Essential and Pro
10.YLI - silk
11. Sulky
12. DMC
13. Guttermann
14. Mettler
15. One basket of misc brands.

All bought on clearance and going out of business sales. I use them all with no loyalty to any of then.

for piecing I use a light grey Masterpiece. For quilting Permacore, King Tut or sometimes what ever I have in the right color!

AURIFIL. Masterpiece also good, and it's straight-wound, so it works well on a featherweight without a thread stand.

Coats an clark for piecing, Fufu for quilting.

I use Connecting Threads for piecing most stuff. If I really need the scant 1/4 I will use Superior Masterpiece.

I have a cone of aurifil in grey, used for piecing ...and mettler for sewing my bindings to the back side of the quilt.,,

OK, you got me. What's Fufu? According to Google, it's food.
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Connecting Threads but I am new so my favorite is also the only one I know.

My SIL bought a kitty litter bucket of thread for $5 a couple of years ago at a yard sale. It's mostly C&C I think. There's not a color not in that bucket. For the most part they are small spools and are fine. I'll use whatever is on hand as long as it's not glaringly off color for what I'm working on. My Viking isn't picky about thread so I've had no issues.

I agree about Aurifil, most miles for the money. piecefully, the old one