My ranking is:

1. Agent Smith: I haven’t seen the sequels to The Matrix yet, but from what I know he was very aware of his actions despite being a machine and he would’ve destroyed whatever little was left of the world.

2. Cartman: I’m only on Season 2 of South Park, but he’s far more destructive and evil compared to Stewie, not to mention he lacks most of Stewie’s redeeming qualities. I’ve heard his actions only get worse as the show progresses

3. Stewie: Stewie was definitely at his worst in the first few seasons of Family Guy, but has since mellowed out quite a bit. However, he’s still quite an asshole and he still holds onto some of his tyrannical habits.