First, do not buy lots of one brand. Buy several different brands. Black is the hardest color to work with because of the dying process. The moisture content is a factor, so keep your thread away from direct sunlight and dirt/dust.
Rayon has a beautiful sheen, but it also fades and does not wash as nicely as poly, but don't resist not using rayon, if you like it, use it.
Listen to what others suggest, but make your own decisions. What one person likes is the worst for someone else.
i have used Baby Lock and Beother machines for 30 years. I have used every brand of machine on the market and have owned every bran, except one. I have never had any problem with any brand of thread. But, I do have my favorites. Love Floriani, Marathon, and Exquisite.