According to Official Guinness Records,
The brightest asteroid is 4Vesta (discovered on 29 March 1807). It is the only asteroid visible to the naked eye. This is due to a combination of the brightness of its surface, its size (576 km, 357.9 miles across), the brightness of its surface and the fact that it can approach Earth as as close as 177 million km (110 million miles).It can reach a maximum apparent magnitude of 5.0 as viewed from the Earth, about as bright as the faintest stars visible by most people.
For a complete list of 1807 records, please visit 1807 Guinness Records in United Kingdom.