Doesn't it matter what type of flight we are talking about? The IFR/VFR thing is an important consideration.

Does an iPad/tablet count as "one" instrument? This would be my first choice because it would give you a ton of information, but I think your question is about one instrument that is part of the plane.

Oil pressure is pretty useless information IMO. How often does oil pressure go bad, how often do engines quit? It's pretty rare if you look at it from a statistical standpoint. Actually I would eliminate all of the engine instruments; you don't need any of them to complete a flight and get the plane one the ground safely. Use your eyes, ears, hands to manage power.

Fuel quantity is not needed. Any pilot worth his salt should know how long he can fly before becoming a glider.

So, if it is IFR my vote is for an attitude indicator. Nobody can use their eyes, ears and butt to fly a plane in IMC without this information. If you keep the plane level and have a good sense of the big picture outside then you have a good chance of getting the plane to VFR and landing safely.

For VFR I would say it is the compass. I guess this is assuming you know where you are, have a paper map and remember how to use it.

Are you going to let us know what your FO's choice was?