It's funny. I built a cross cut sled and never use it. Well almost never. I cross cut on the radial arm saw 99% of the time. So, I had to think on this one and my most used is my bench(es). Not exactly a jig but I do everything on the two benches.
As far as true jig, it would be a toss up between the accessory fence(s) for the table saw/miter gage(s) or the circle cutting jig for the bandsaw. I think the accessory fences would win though. I have several that I use quite regularly.