Mostly you should match your needle to your thread and the type of fabric you're sewing. Since I mostly sew cottons I use a 75/11 quilting needle for piecing. I generally use either Aurifil or Prescencia threads to piece with. For quilting I use a wider variety of threads so I use more sizes of needles. For most quilting I use either 80/12 or 90/14 topstitch needles. The topstitch needles have a larger eye and work better with the heavier threads.
For cotton fabrics the sharp needles work best. For knits you'll need the ball point needles. You're supposed to be able to use universal needles on either type of fabric but I've found they just don't work all that great for either one.