The plank isn't everyone's favourite, but there's no denying it's very effective for building core strength, full-body stability, and endurance. The only exercise that's potentially better for all the above is the plank row.
This variation uses a set of dumbbells to "combine upper-body strength with core stability", says Mark Harris, a level 4 personal trainer who works with Mirafit. While one arm stabilises you, the other lifts the weights from the floor.