Heading, Rate of Turn and Rudder sensors for all autopilot types.
All autopilots need to know what is going on around them. Rudder sensors, whilst some autopilots manage without a rudder sensor, having one is always better. If a vessel doesn't have a rudder sensor, the computer has to wait for the boat to respond to the rudder position before deciding whether to add more rudder or less. If the rudder position is known precisely to the computer, the computer can better guess how much rudder to add in the first place. In many cases having a rudder sensor also allows a rudder position indicator to be shown. This can help with maneuvering in dock and also with sail triming, a centred helm makes for a faster boat!
Compass sensor, the most essential component of an autopilot. The autopilot needs to know what direction the boat is pointing in order to steer the boat on the desired heading