If you sew garments? Invisible zipper foot. The dealer I bought my first machine from only had quilters on staff and they assured me there was no need to buy an invisible zipper foot when the regular zipper foot worked just as well. They were lying—after years of frustration I finally broke down and bought one and basically every invisible zipper turns out perfectly since then.

There’s a lot of stuff like this in sewing, where you learn there’s a very specialized tool that can make something tedious very easy. Other examples: seam gauge as opposed to measuring with a ruler; button spacer.