Angelus was certainly the most diabolical big bad, simply because he made his attack on Buffy and her friends a personal thing, above and beyond any desire to destroy civilization as we know it. I think that he was also the only real threat that Buffy actually hated, and for that very same reason. Whenever she fought, confronted, or even thought of him, it was impossible NOT to see Angel, her first love, and to be reminded that she was directly responsible for what had happened to unleash his evil side. Add to all of that the fact that she had to face Angelus at a very early stage in her career. The only big bads who she had faced previously were the Master and Spike, both of whom had simple straightforward agendas: seize control of Sunnydale and set up their own little kingdom there. To them, she was simply an obstacle to be overcome, an enemy to be defeated. They were enemies, certainly, but it was far less complicated. They were predators, attempting to claim territory. They never really bothered to make the fight personal. As Spike said in Season 7, "She was the Slayer. I was the vampire. That's the way the game was played." And, to a certain extent, it WAS a game, played for very high stakes. The Master, Mayor Wilkins, Adam, Glory, Dark Willow, even the First Evil. They all played the game according to the rules, no matter how diabolical and convoluted their strategies might be. Angelus was the only one who cheated.
