Joffrey is a cunt. It's easy to loathe him. He is literally a psychopath in the vein of the mad king.
He would probably go down as FAR worse than the mad king if he actually lived that long. Can't wait till karma kicks him in the nuts.
Littlefinger is an asshole. He's good at what he does, but my god do I hate him as a person. Conniving bastard who only cares about himself. At least Varys seems to be a 'good' person and has good reasons for the things he does. He's not simply looking out for #1.
Also, I think Cersei is a way more 'vile' person than Littlefinger. She is an entirely terrible person with zero morals.