I did like Becky Rosen’s introduction scene in 5.01 “Sympathy for the Devil” where she is seen composing fanfiction and then, when she first meets Sam and Dean, she gently strokes Sam’s chest. It’s beautifully hilarious.
Tracy Bell’s introduction scene was also pretty cool and sassy. In 9.02 “Devil May Care” the young hunter uses her feminine charms and attractive body to get close to a vampire in order to take him out. A great strong female character shown easily in her first minute of screen time.
I also really loved Eve’s entrance in 6.12 “Like a Virgin”. Eve’s virgin sacrifice and vessel rises from a burning pit, in just a pure, white dress, floating eerily towards us, asserting instant authority and letting us know that sh*t is about to go down.
However, when I first watched 3.01 “The Magnificent Seven” which debuts Demon Ruby’s first appearance, I was surprised, intrigued and dying to know more about this mysterious, blonde bad ass who just entered the building, saving Sam’s life for completely unknown reasons. The stranger bursts in on the action with a new weapon that neither Sam nor the audience have ever seen before. She takes out the few Demons attacking Sam as though it were easy. At the time, it was so different and interesting as we have never seen a knife that can kill Demons before and, after exchanging only a simple line with Sam, “I’m the girl who just saved your ass…See you around, Sam”, she mysteriously disappears into thin air which is also so new and leaves the audience in the same boat as Sam; curious and eager to find out who this girl is. She really makes a big entrance and a massive impact on me.