This can often be fear, disgust, or sympathy. What makes for a particularly memorable villain, however, is if they make the audience genuinely respect them. This can be through impressive achievements, genuine virtue, or simply an impressive demeanor. Plenty of antagonists in film are respectable, but some really stand out in this regard.
In all of film, there isn't a single villain as iconic or beloved as Star Wars' Darth Vader. Aside from the high-status Star Wars enjoys, this stems from just how many respectable qualities Vader has. Even with his villainous, cruel, and even petty actions, he manages to stick out in the mind of anyone who has seen the films.
He's immensely intimidating and dangerous, being almost unstoppable throughout the franchise. He's responsible for some of the franchise's most iconic moments. However, he also has a dark and dry sense of humor, and undergoes one of the most famous redemptions in film history. Everything about Vader engenders respect, cementing him as a fan favorite.