I liked how the Legion was used in Harmony of Dissonance the most. The contrast between the Legion Saint and the Legion Corpse was cool, and they are fought differently. I have to give it to the Legion Corpse. The Legion boss has always been really creepy, looking like a giant floating orgy of soulless bodies, buy the Legion Corpse takes that and makes it creepier. Legion Corpse is just a sick, decaying version of Legion, which you "excite" by hitting it near the center, then as the zombie-vagina-like part opens, you find that it is being manipulated by a skeleton in the center. It's creepy, disgusting, and awesome for Castlevania!
Legion from AoS was another interesting take. Looking carefully you can see a creature's face withing the mass of bodies as well as two hands. The creature looks like it's in a fetal position. And once you open it up you can see the fetus as the core.
I never noticed that before! Cool!