It would be the Chart-Plotter. (I can hear the gasps and groans as people read this.)

If I could have only two I would add the depth meter... (If I was outfitting a new boat today I'd add a fish-finder sensor to the C-P and skip the typical sailboat depth meter. It's much more useful.)

If only three I would add the wind speed/direction ... (very helpful especially downwind and at night)

If only 4 I would add the speed-log. (Speed-log and compass together became "secondary" instruments with the advent of GPS. That's why I leave my paddlewheel out most of the time.)