Aurifil Threads, which colors are a must have.
 

I love Aurifil! I guess it depends on what you tend to make. I use a lot of soft yellows, creams, white, black, gray, tan, blues - especially turquoise. I love the variegated colors too.

There are some others colors that blend in really well. I don't have a spool with me right now, but it is kind of a cinnamon color. That one works well for many piecing colors. I like the light and darker grey too. I find I don't use much of the really light colors. I tend to piece more medium colors than really light, so I use more of those colors.
Have fun shopping. With the thread being thinner, it goes a long way. It's really good for the actual quilting too.

Look at your fabric stash. Buy those colors.

i have (on hand at all times) light and dark gray- cream- white, black, taupe (natural)
then i keep adding colors- each time i place an order for re-stocking the basics i add a couple new colors- since i work in many, many colors regularly and i never know when someone is going to call-and say i want bright green thread - or what do you have in purple---i'm trying to have a good selection- when i find a special with a 'collection' of colors i buy it- i'm up to 60+ colors now- :)

i like it to just finished quilting a quilt with a dark brown and light tan varigated worked well since the quilts was tans creams browns and reds. am putting a quilt on the frame that im going to be quilting with a red orange yellow varigated. i like aurifil not much breakage and not to much lint i was using some other threads and had alot off breakage.

If you're in Canada, there's a great buy of a "Jelly Bean Doorcrasher" - 66 spools Aurifil cotton mako 50 wt for $78 (discounted from $214). A friend and I were planning to buy - but the shipping to the USA is $20.

http://www.tristan.bc.ca

I tend to use a lot of the cream shades and black for piecing. I have some other colors I bought for fun.

I use tan and silver for most things but have slowly started buying some of the other colors. Love their thread!

20 dollars dosen't seem so high for shipping these days. I just bought a book and it was 4.95 for a little book. I would go for the thread, it would still be less than 1/2 price, can't beat that!