Complete Monster-Marvel Multiverse (UPDATED)
Part II of my Complete Monster lists, this time for Marvel ones. Updated from my last iteration.
As the name implies, a Complete Monster is a villain who is pure, irredeemably evil, sufficiently heinous in relation to their peers, and bereft of many moments of genuine kindness, compassion, or remorse. There is ultimately nothing sympathetic about these characters, though there is a bit of a divide between the ones who can at least be called "cool", and the ones who just aren't. Either way, they're still pure evil.