Mars InSight will be the first to detect seismic activity on Mars’ surface, first to measure rate of heat transmitted from interior, first to dig nearly 5m down, first to measure magnetic fields on Mars’ surface, and first to use a robotic arm to place instruments on the surface of Mars (assuming it lands of course…)
If you want to watch the InSight landing “live” (with 4-minute speed of light time delay), go to: https://ve42.co/insight