Mostly Aurifil cotton mako 50wt (I bought a huge stash on clearance from Tristan Threads), Masterpiece by Superior (my stash is all the older 2 ply, haven't tried their new 3 ply version), and Presencia 60 wt cotton. I like them all. I wouldn't by the Aurifil at full price, though, too rich for my blood.
Like some other posters, I have a huge cone of warm grey Presencia 60wt that's fab for precision piecing and blends well with most fabrics.
Connecting Threads Essentials cotton is pretty ok and very affordable. Much lintier than my favorite brands, but otherwise fine.