I like Isacord but I've also bought generic thread from Threadart and it works great in my machines. I've found that some of my machines don't like certain threads. My Brother doesn't like Madera, my Janome isn't fond of Sulky. The all seem to consistently like Isacord so that's what I stick with.

My Viking seems to like everything - when I first started machine embroidery I bought the chest of assorted Madeira threads to get me started with lots of colors so I could do pretty much anything I wanted - as I use up a spool I add back the needed color in whatever brand is on sale in my area.

My Babylock Ellegante will take any thread I want to use, so rather than buying by brand as a criteria, I buy by thread content, and I use polyester most all the time rather than rayon, as I make lots of decorative kitchen towel sets and napkins, table runners, and place mats, which by their use, require frequent washing. If I am going to be making something like a wall hanging, etc., something that doesn't require frequent washing, then I use rayon or a combination of rayon and the polyester threads as I probably will have more color choices by using the two thread types. Polyester is also nice and shiny, so I do not find any great noticeable difference between the two types used within the project