I like all of the ones mentioned already, especially Friendship Star. But if you want the easiest of all, except of course the plain square, nothing beats the Stretched Star. It also makes for a beautiful quilt. You just cut a square in a plain fabric, and stitch a little triangle at two opposing corners. When you've done this to as many blocks as you need, sew them all together, arranging them so that the bright corners touch each other, and Bingo! you've got a lovely quilt top. You can use whatever scraps you have around for the corners and get a nice scrappy look.
At first the little corners look like small butterflys, but eventually you can see the stars forming. It's a fun, quick quilt.