Assuming the frame is a given:
"Engine"
followed a long way behind by:
Fork
Tires
Wheels
Brake shoes
Saddle
Tubes
Stem
Shifters (brake levers/hoods included)
Bars
Brake calipers
Cranks
Pedals
Seat post
Bearings (upgrade to ceramic)
Chain
RD
FD
Cassette
Cables
Bar tape
Bottle cages
Skewers
Bottom bracket
Headset
Screws (upgrade to titanium)