Bottom Line, King Tut, serger thread, Coats Star quilting thread, ThreadArt embroidery thread, Connecting Threads Essentials cotton, Soft Touch cotton, 30wt generic cotton thread, Isacord, Aurifil 50 wt & 60 wt, generic poly bobbin thread....
Use the threads that give you the effect that you want on each side of the quilt. This might be the same thread top and bottom, or you might mix two different threads.
Some people are against using poly thread in a quilt, but I've gotten to where I use what gives me the look I want without any regard to the fiber content. The one exception is that I won't use fiber that doesn't stand up to laundering or heat, so rayon and nylon are out, for me.
It's mostly what you prefer and mostly subjective.
The way I look at it, if the last parts of my quilts left standing are the seams and quilting and the fabric is disintegrated in fifty or a hundred years, so be it. I don't believe that the difference between poly threads and cotton threads is actually great enough to cause that, but people more knowledgeable than I might know better. :)