YLI is my favorite monofilament. It is a nylon thread. I have tried polyester monofilaments, but find them less invisible than YLI. However, as fine as YLI is, it is still a little more "wirey" than I like for quilting (I use it for machine applique). What I prefer for invisible quilting is Bottom Line thread, which is 60wt and very fine and supple. The silver thread color (#623) seems to blend into everything from very light to very dark.
SewArt monofilament was the brand that Harriet Hargrave preferred, but I think it was discontinued years ago.