Interesting Villains vs Realistic Villains

One thing that professional writers like to say is "If you're making a good character, look for something bad in them. If you're making a bad character, look for something good in them". 

This is how you make interesting villains. 

Gollum is an interesting villain. He's a mixture of pure, relentless, unstoppable evil and a really, really old guy who misses his family, with some comic relief thrown in. 

Dragon Highlord Verminaard is, frankly, quite boring. His enchanted mace is a more interesting character than himself. He's just an evil cleric who rides a dragon and wants to kill you. Why? Because he's evil. 

Lord Soth is more interesting because he's an undead horror, but also he used to be a knight and he still follows the noble and chivalrous code - to an extent. 

Strahd von Zarovich (specifically as depicted in "I, Strahd"), Kitiara, Raistlin - you get the idea. 

And when making a realistic villain, you look for something else that's bad in them. 

Wormtongue is a realistic villain. He's a traitor, and he's a coward, he lies and steals, he harrasses Éowyn, and he stabs his boss in the back. 

Greg Stillson in Stephen King's The Dead Zone is a realistic villain. He's bad in more than one way. When he uses a baby as a human shield, it feels natural and organic to the character from the way he behaved from the start. 

If you look for examples in real life, Hitler would make an interesting villain - a decorated soldier who volunteered to go to war, and a loyal friend, as well as a genocidal monster. 

Putin would make a realistic villain - lying, corrupt, cowardly, and abusive, as well as a genocidal monster. 

Both kinds of villains can work well in your stories.