If you're playing in-game, Bulk Up is one of the better Corviknight sets, however that's only because Corviknight's offensive movepool is small. You generally want to brute force your way past everything with immediate power, however Corviknight gets few useful offensive moves in game.
Competitive is a different story, and the true answer to this if you're playing competitively is with what your team needs. Does it need a Defogger? In that case, the Defog set is generally better. However, if you already have one, Bulk Up can work pretty well. About running HP and Special Defense, Bulk Up Corviknight usually runs Body Press so you either run HP and Defense with 4 Special Defense EVs, or 252 HP and a large amount of Defense EVs with the rest piled into Special Defense to be more bulky on the Special side. I feel like 252 HP / 252 Def / 4 SpD is generally better. Bulk Up also fits on balance better than on stall or semistall.