Really important

  • Helmet
  • Bike
  • Cycling shoes (If you intend using them)
  • Inner tubes
  • Emergency patching kit
  • Pump
  • Tyre levers
  • Multi tool including Allen keys
  • Saddle bag
  • Lock

Clothing

  • Bib shorts
  • Socks
  • Jersey
  • Base layer
  • Gloves
  • Sunglasses
  • And depending on the conditions
  • Rain jacket
  • Gilet
  • Arm warmers

Nutrition

  • Water bottles
  • Electrolyte/carbohydrate tablets
  • Energy bars/gels/foods – Fruit bars, bananas, gels, cheese sandwiches, Mars Bars or whatever works for you
  • Cash – Enough cash for a decent meal and a taxi.
  • Debit card – In case you run out of cash.
  • Photo ID and address & emergency contact details
  • Mobile phone
  • Navigation device/timer

Miscellaneous

  • Sun cream
  • Organisers telephone number
  • Saddle cream

List adapted from: 
http://www.sportive.com/cycling-training/499910/what-to-take-on-a-sportive-a-checklist-for-riders