I'm with Kev, naked without a camelbak.
Problem is though, get something like a Mule, and you end up stuffing it with stuff you don't really need.
Mine is stuffed with;
Tube or 2
Puncture kit
Pump
Tyre levers... proper metal ones
Multi allen key
Screwdriver for mech screws
Chain tool
Zip ties
Electrical tape
Museli bar of sorts
Phone
Waterproof, depending on weather
Keys
And probably a whole host of stuff i've forgotten!