This one is later than planned. I've got a project at work that's extremely time/detail intensive, and I've been playing Solo Dad while my wife travels on business. So, these updates have taken a back seat.
That said, after spending three hours on the aforementioned project, I needed a mental break, so here we go with possibly the evillest dude in Classic Enemies: Dark Seraph.
This guy is just terrific. From a rules/construction standpoint, he's raw power with very little in the was of efficiency, because points are for players, not GMs. He's mainly a very tough hand-to-hand fighter, with high defenses as well as flight and the ability to turn desolid. He can also stand off and summon a massive (18d6!) explosive thunderbolt. On the non-combat side, he's a powerful telepath, which is great for getting into the heads of the heroes and using their innermost thoughts against them.
Personality-wise, he's the vilest of the vile. Even Black Paladin looks at Dark Seraph and says, "Damnnnnn..." GMs can count on Dark Seraph to do the absolute worst, to the limits of what your campaign can stand. The heroes should fear him, the players should loathe him. Best of all, he works on both a mystical front as well as a basic opposition to good. He's equally suited to fighting your Sorcerer Supreme or your Star-Spangled Patriot.
I first used him before 4th edition without much forethought. If I remember correctly, I needed a magical villain and I chose him rather randomly out of Enemies III, without really looking at his powers or tactics. On his first action, I dropped the thunderbolt and basically one-shotted the the heroes (I think one was still standing and DS mopped him up fast). Since then, I've been more circumspect about his use, putting him behind occult conspiracies and using other magical villains as his lackies. One notable campaign kickoff started with him in custody, with the heroes coming together to try and foil an attempt to bust him out of his magical incarceration. Good times.
On that note, I've got to get back to work. These systems aren't going to inventory themselves.