Like most everyone else, the most frustrating quilting notion/gadget for me was the SEAM RIPPER ... first week I had my machine I broke the one that came with my sewing machine. So I "borrowed" the one that came with my daughter's machine. Worked okay, but was so small that I've misplaced it/lost it (it didn't grow legs and walk off so just where could it be? Still haven't found it). So now whenever I'm out and about and I see one on sale, I buy it and try it - so far, my favorite is the Seam Fix - it's got a good "grip", works great, and the best part is that rubbery top that picks up all those little threads.
My 2nd most frustrating tool were my old rulers - I had one that had neon green lines that literally gave me a headache; another ruler that was frosted and hard to see through, and one with thick lines that gave me inaccurate results. Mind you, all three were different brands, as well (which I learned is a no-no!). So I got rid of them and with the advice of this board forum, I invested in the CREATIVE GRIDS rulers and the June Tailor Shape Cut Plus slotted ruler (and new rotary cutters, too). LOVE, LOVE, LOVE my new rulers & cutters - it makes cutting so much fun! My 9-year-old daughter's enjoying it so much, too, that she often asks "need anything cut today mom?"