I have some Aurifil variegated 50/2 thread that I really like for machine quilting. (Aurifil 50/2 is what I am using for everything right now, including piecing.) This is a relatively fine thread that is good for medium and close quilting. If you are quilting farther apart and/or want the thread to show up more, then 50/3 is a more common weight to use. I think Superior Thread's King Tut is that weight. King Tut is a very popular machine quilting thread and comes in a wide variety of plain and variegated colors.
The first number in a thread indicates weight; the higher the number, the finer the thread. The second number is the number of plys (strands that are twisted together to make the thread). 60/3 thread is finer than 50/3; both have 3 plys.
I like Aurifil because there is virtually no lint from it. It is a very smooth thread. Also, because it is fine, the bobbin holds a lot more than say Mettler 50/3. This means I don't have to stop and re-wind the bobbin as frequently. For piecing, the seams lie flatter than with a thicker thread. For quilting, 2-ply means it's not quite as strong as 3-ply, so is best suited for medium and close quilting. Quilting lines that are farther apart will take more stress, so a 3-ply would be stronger for that purpose.
When you want the quilting thread to really stand out, as for decorative quilting, then you probably want to look at a 28-weight thread (something with a lower number than 50).
That is my inexpert lesson on thread for the day!