I decided to rank the villains based on the big bads of each season of the TV show. A couple of times, these shifted part way through the season so I included multiple people. From greatest to the least great.
1. Spike/ Drusilla/ Angel- Season 2, the writers and actors went to work making us care about these crazies Spike and Dru. Then they upped the ante by having a good guy Angel join them. How more invested could you be in villains like that?
2. The Mayor- I love the Mayor. For being invulnerable, he was vulnerable and cared about things while being genuinely scary.
3. Dark Willow- Willow was another instance of us caring because we loved our sweet Willow so much. She had quite the reign of terror but was not evil for a very long time.
4. The Trio- They were entertaining. However, there were also largely ineffective. Warren was pretty gross. Love Andrew and Jonathon though.
5. The Master- I enjoyed the Master. He was our first season long big bad. He was a good mixture of scary and grumpy old man. Not unlike Voldemort in the last "Harry Potter" movie.
6. The First/ Caleb- Caleb was another kind of gross villian but he was played by Nathan Fillion which earns him love. The First was scary but also incorporeal. I know Amber Benson didn't want to taint the character of Tara by coming back in the last season as an evil presence but it just made the First look kind of weak by bringing a dead stranger to come back and try to talk Willow into suicide.
7. Glory/Ben- I found Glory entertaining and the situation with Ben was kind of tragic. I just didn't feel all that bad for Ben which dragged things down.
8. Adam- He was definitely interesting looking and had his good qualities. Just not my favorite villain.
9. The Anointed One- I liked that the anointed one had powers over vampires, was a little buddy of the Master and was generally adorable. He didn't do much of anything though.
10. Professor Walsh- Same problem as the anointed. She was definitely over invested in the lives of her employees but other than waking up Adam, she didn't do anything.