I only use 100 percent cotton thread for piecing and quilting; like others have said, if your threads are poly/mix I'd use them for basting etc.
For piecing, I exclusively use black, white, grey, cream and tan/beige. I use Aurifil.
For quilting (hand or machine) I have white and cream, also a goldy/tan variegated one that gets used a lot. Otherwise, I buy quilting threads as I need them for specific projects so have built up a modest collection. I do like the variegated ones though, for Free Motion especially. I get my quilting thread from a UK company mail order - their own brand, so not much help to you, I'm sorry! I'm believe Superior Threads are the best though, I think they're the only brand who use extra long cotton fibres and might change over to those next time I have to order. :-D