My favorite thread to machine piece with is Aurifil 50 wt. If you've never tried it, I highly recommend it. It's strong and has almost no lint. Of those threads I've tried, it's definitely my favorite. But I'll admit it can be pricey. At my local quilt store, it's $10.99 for the solid colors. There's a lot on a spool though and it lasts quite a long time.
I recently ordered some online for $6 a spool from Sue's Sew EZ Designs. Same stuff, just a much cheaper price. She also ships up to 24 spools for $5 (continental US only I believe). I'm mentioning this now because the sale price ends at the end of this month. I've ordered from her several times before and have always been pleased with her customer service. Just passing on a good deal in case you're interested.
Note: This is a completely unsolicited and unsponsored post.
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