I mainly use Glide 40 wt for the top and 60wt on the bottom. I like them because they are polyester so don't shed fuzz like the cottons, the sheen, the look, and the price. I've also used Isacord 40wt, Superior So Fine 50wt and Superior's Monopoly Invisible and they've work well, but find Superior's price point considerably higher as is Isacord. Like others have said, it depends on the project and the look you're going for that will influence thread choice.