My Clover stitch ripper. Maybe favorite isn't the best descriptor,rather most used! If you're not ripping you're not sewing.

Twenty five or so years ago I took a class on mastering bias at Wayne State University. I didn't know anything about Mr. Kleibacker - I had read about him in a magazine and found that he was to teach this two week summer class in
Detroit that I just had to take. He showed us how to create muslins and then stitched us into them to fit them perfectly. When it came time to remove mine he discovered that he had stitched it to my body. He was amazed I hadn't complained at all. He was so generous with his time & expertise - what an honor to have studied with him.