I've been using mainly mettler for years, living in Switzerland that's what's most available. Since finding TQS I've learned about lighter weight threads and have been buying wherever I go and trying out various things... My favorite at the moment is superior, bottom line for piecing and the rainbow's and perfect quilting for quilting. I'm also quite happy Weith the YLI 100% long staple cotton... What I use mainly depends on the project in question. I'm still waiting for my first order of superior metallics - not that they were long in shipping, but to save on postage and customs fees, I have them sent to our office in the states and whenever someone travels from Miami to our office in switzerland, they carry a little something with them for me... Next week is going to be prezzie week for me... As I'll have three nice people traveling my way early november, I'm trying to figure out what I 'need' for my birthday.
The key is that you really should try whatever nice threads you get your hands on and find out what works best for you and your machine