well, i'm a notion junkie. the only one that i've come across so far that is a blooper are the curved safety pins that are supposedly especially made for quilting. they aren't very sharp so i had to really work to get them through the sandwich. switched to the brass #1's and i'm much happier!
my favorite notion: all of them! i'm forever picking up a new ruler or something that i will 'eventually use'. my current search is for the 'fork pins' that you use where your seams meet that hold down both sides of the seam (i'm sure that's clear as mud!).
great question!
patty