It's difficult to answer this question because one part might be much more aerodynamic than an equivalent, but when installed on a bike, the advantage largely disappears. I can imagine in some edge cases a freestanding part is more aero, but when installed on a bike, the overall system would be less aero.
GCN recently did some wind-tunnel tests and according to them (somewhat to my own surprise) one of the biggest improvements you can make is to your helmet.
Some people use the "hairsine ratio" as a metric for grams saved per dollar spent; you can do something similar for watts of aerodynamic drag saved dollar spent, and this is what the aero assistant at Aeroweenie attempts to do (it uses seconds over 40 km, not watts). It is general--it's not comparing specific brands of stem or whatever, and it doesn't let you configure every part of the bike, but it picks all the low-hanging fruit (note: this hasn't been updated in a while, and it disagrees about the helmet); this also adds low rolling-resistance tires to the list of improvements. There are lots of other interesting resources at that site.