The rotary cutter hands down. I'm called the Queen of Gadgets in our family and it is a title rightly deserved because I love gadgets, kitchen, office, sewing, etc.
So at the Sewing Expo yesterday I purchased the seam ripper that will never need to be replaced. How many times have you heard that one? Anyway it is made by Wahl, the hair clipper folks, and I tried it on one of my practice blocks and OMG it just might be the real deal. Updates to follow.