10 Hygiea
At most oppositions, Hygiea has a magnitude of around +10.2, which is as much as four orders fainter than Vesta, and will need at least a 4-inch telescope to resolve. At a perihelic opposition, however, Hygiea can reach +9.1 and may just be resolvable with 10x50 binoculars, unlike the fifth and sixth biggest asteroids 704 Interamnia and 511 Davida which are always beyond binocular visibility.