i like to keep things as simple and lightweight as possible.

for me, the only real essential is a really fast road bike. other things that are nice to have:

spare tube
CO2 pump and cartirdge
tire levers
multi-tool
water bottle and cage
lights (small lightweight blinkies) i ride in the city, so illuminating the road is not important for me
backpack for change of clothes and other small personal items

that's it for most days. for inclement weather i have my old mountian bike that is equipped with fenders.