C-type asteroid
The largest unequivocally C-type asteroid is 10 Hygiea, although the SMASS classification places the largest asteroid, 1 Ceres, here as well, because that scheme lacks a G-type.
31 Euphrosyne
The mass estimate of Euphrosyne in Baer (2011) makes it apparently the 5th-most-massive asteroid, coming after only the "big four". It also has the highest estimated density, indicating that it is a solid body like the other largest asteroids. However, all large asteroids with comparable densities (16 Psyche and 532 Herculina) have very large uncertainties, so both the mass and density are likely to be lower than the median estimate.