What kind of thread I use depends on what I'm doing. The one I'm working on right now had a lot of piecing and quilting.
So I decided to use Maxilock serger thread so I don't go broke just buying thread. If it's on sale I can get two 3K cones for $5. I believe it's cotton. I can iron it at any rate.
What I'm working on now has cream backing so for decorative topstitch I'm using tan Coats & Clarks with white serger thread in the bobbin.
Of course the thread doesn't fit on my Singer 9022. I hung the cone upside down from the chandalier with a rubber band paper clip binder clip affair. It's cheap, it works so I'm happy.