The easiest and quickest (under 30 seconds) would be your entire handlebar. They'll just have to use an allen key and loosen 3 bolts. Then use a small wire cutter (which fits in a shirt pocket) and cut the four wires (2 brakes & two for shifting).

He'll walk away with your handlebar & everything on it:
-computer
-light
-STI brifters
-stem

And it'll take less than 30 seconds. That's why I don't own a lock. My heart will drop if I ever come back to my bike without its handlebar.