Assuming I'm just some random normal/baseline guy in the MU my picks are:
The Sentry/The Void: He's unpredictable and somewhat arbitrary in how he "balances" the scales.
The Lizard: He's just predator that hunts for people. I find him scarier than guys like Carnage, because Carnage typically just offs people. The Lizard can't be reasoned with and may not finish you off right away, not as a means to prolong your misery in and of itself. He might just be full and save parts of you for later.
Magneto: He's self assured in his righteousness, and adheres to a "frontline is everywhere" and ends justify the means philosophy.
The Kingpin: He will target you and your family over BS. If you #### with his money at all, he may view your entire friends and family network as disposable.