Aurifil all the way for piecing and most quilting. The same neutral colors mentioned. Depends on the quilt for the quilting. Occasionally YLI variegated for quilting, great colors.

For hand applique I use YLI silk.

Three colors of thread is all that is necessary for piecing.
I always have a white/cream, a light tan, and a medium gray. with these three you can piece light, medium, and dark fabrics.
Always buy a good quality thread. I usually use Mettler. But a local quilt shop had Masterpiece by Alex Anderson on sale. So I've been using this thread lately. Cheap thread will leave a lot of lint in your machine. If you are going to do Applique,you will need more thread colors to match your fabrics.

I use a good quality thread for piecing in Grey, beige or black. One of these colors will work with almost all fabric colors.

I asked for a "frosted donut" for christmas. For those of you who don't know what it is - it is 36 bobbins filled with a rainbow of Masterpiece thread - great for applique. Most of my family could not begin to understand how I could be so happy over thread!!

I use grey or off white for most piecing.

For most piecing, I use Connecting Threads -- my machines all like it, and you can't beat the price. Also, I use a lot of dark grey or navy blue for piecing (on darks) and a butter/cornsilk color for piecing on lights. If you've ever had to "un"sew on black fabric, you'll appreciate how easy it is to see the navy or grey against the black background.

I've sewn for years and have a large collection of colors. When I am quilting, I typically use a neutral color as it blends well with most. I'm fixing to start a black, red and white quilt and will probably use black for most of it. One thing I try to keep in mind when I am getting ready to piece a project (especially when I am machine quilting) is to have several bobbins already wound. If I'm using my Singer it can't wind a bobbin indepedent of the sewing functions and it is really frustrating to have to unthread the machine to wind a bobbin. :|

AMEN to the frustration opf having to unthread to wind bobbins.

Ditto what you said patrice. I have one of each and many doubles/triples of the large purchase seperate spools (For quilting on thier kits I bought.) Makes it easier to find the right color that way.
I also use Aurifil , King Tut and other Superior threads. It depends on whether or not the piecing is complicated with lots of intersecting lines or not. The quilting thread is dependent on how nice the quilt is. If I have invested a couple hundred dollars or more in a top, I want the best I can get my paws on ...LOL!
Drag around quilts, I use a medium grade thread my machine likes.

I love the thread from Connecting threads. The color collections are great. I use the neutral colors for piecing.