Spike's entrance early in season two was a big turning point for the show. Up to this point the vampire population of Sunnydale had been mindless thugs with little personality. Luke and Darla were a little more developed but they didn't stick around long enough to make the impact they deserved. (Thankfully Darla would be back in a big way in spin off show Angel). The Master was a lot of fun but he verged on hammy once too often.
Spike was a different breed altogether. A witty, sadistic Billy Idol meets Sid Vicious-type, James Master's iconic villain was a hit from the moment he appeared on the show. A killer of two slayers with a mysterious connection to Angel and the deranged yet very dangerous consort Drusilla, Spike was a very new threat to Buffy and paved the way for other villains with depth like The Mayor, Faith, Glory / Ben and of course Angelus himself. Even the way he makes his entrance is epic.
Crashing his blacked-out Desoto into the 'Welcome To Sunnydale' sign and stepping out to a heavy rock guitar riff with his striking bleach blond hair and black trench coat, smoking a cigarette and speaking with that distinctive accent, this was a vampire that was both cool and intriguing. (It's brilliantly spoofed by a drunk Spike in season three's Lover's Walk). Spike immediately attacks the school, almost exposing Buffy as the slayer to her mother and kills the 'annoying one', making every fan cheer. For a first episode, none did it better than Spike. It's not surprising they kept bringing him back for more.