I also struggle with this, so I don't have a proven solution yet, but here are my ideas:
1. Go over the top. Cheesy overacting is good. You want it to be memorable, not subtle method acting.
2. Look up some scenery chewing villains from your favourite Hollywood movies. Look up some quotes from Alan Rickman's characters and other favourite villains. Sure, a direct quote may not be original, but your players will hopefully immediately think of that famous villain and assume your villain is like that. Leverage pop culture short-cuts.
3. If you've got two evil villains, the risk is that they will be too similar and the players won't tell them apart. So make them opposites. The son is brutal in-your-face, wants to see the PCs suffer and would rather hurt them than kill them. If they ever get captured by him, they will survive, but with a new reason to hate him. The dad is the opposite: subtle, friendly, but utterly treacherous. He may be brutal and cruel to people he controls (underlings, no doubt his son when he was young, which is where the son learned his cruelty), but will be super nice and sweet to people he needs, people who might be a threat, or people he's unsure of. Put them on the wrong foot, then betray them. Or sell out someone else to placate them. The son is openly cruel and honest about it, the dad plays nice but is secretly worse than the son. He talks politely and smooth, but the things he says contain subtle things that are really creepy.
