I also was told by my dealer that Mettler was the best. because of course that is what they sold. But I have two Bernies and I use any and everything in them. Mostly for quilting I use he C & C I get in the large spools at JoAnn's when it is on sale for half price. The 100% cotton. My machines are so cooperative.

Yes, it is a funny thing about dealers. I have an older mechanical Kenmore and when I took it in for cleaning I was told never to use Coats and Clark in it because it is terrible thread. It just so happens that he sells every brand except C&C. Hmmm. That Kenmore has been going strong on C&C for years. Hmmmm.

so I guess the answer is to "ask your machine".
I have Bernina, Janome etc. All thread appear to work well. When I see a shop closing and they have the older Coats and Clark I buy all they have left.
So guess there is no sure answer to the question.

I have an older Bernina 1260 and it uses all types of thread just fine. I use mostly Aurifil thread because I bought a case of neutral light and dark colors on big cones from Tristan Threads at clearance price several years ago. I'll never use all that up! All newer threads are manufactured better then they use to be. The Bernina dealer here that has Berninas set up for her classes uses the big cones of no name cotton thread on the machines. She sells all brands of thread and doesn't push any one brand. She will tell you which thread is considered the best for what sewing you are doing and you decide which brand you like to use.

I'm new to quilting, and haven't sewed in 35 years or so. We used to always use Coats & Clark thread. What's the matter with it? I've heard it sheds more lint, but I just clean the machine regularly. Why are other brands better?

The older machines need to have the thread tension loosened slightly to handle Masterpiece thread.

When piecing using a 50 wt thread like Aurofil or Masterpiece it doesn't take up so much room in the seams and they will lay nicer. It doesn't seem like that little bit should make difference but it does.

My Bernina has loved every thread I've thrown at it. I use coats and Clark, connecting threads essential, isacord, yli, mettler, superior and probably others.

Dealer says isacord, mettler are best, but the machine does not seem to be picky.

Aurifil is my "go to" thread. I'm trying to use up my Mettler Silk Finish cotton, but since I started using Aurifil a few years ago, I really don't like it. Compared to Aurifil, it seems "slubby." Superior and King Tut are others I like. Other brands didn't necessarily work better in my machiine, I just prefer the look and the beautifully flat seams that I get when using the Aurifil. And when I bought a multiple-spool package on-line, the cost for the large spools is quite reasonable.

Monofiliment thread in the top gives me very little trouble, having followed a professional quilter's tip to use a drop of Sewer's Aid on it before using.