I like Coats Dual Duty and XP - never had a breakage problem or slubs. The stuff I have is smooth and strong and comes in beautiful colors - most of it is from the 70's and 80's, so it's 30-40 years old. Easy to do hand-sewing with it, too.
Every time I buy a "new" vintage machine in a cabinet, at least one of the drawers is stuffed with spools of thread. I keep the Coats Dual Duty and XP because it will usually sew just fine.
I also use Star size 30 cotton quilting thread - very nice thread, smooth and matte.