The turkey film it came from, its idiotic premise and my unshakeable belief that it was just another Sweet Valley High kept me from watching Buffy the Vampire Slayer until halfway through its second season. As I reached the stunning two-part climax of the season it hit me that the show I looked forward to more than any other was a teen drama about a schoolgirl who fought vampires. I wouldn't miss another episode, right up to its hysterical climax six years later.

Watching for the first time since 2003 – when I viewed the show's entire run in mourning after the series finale – what strikes me is how important Sarah Michelle Gellar was to the show. It sounds dumb but from an acting point of view I always focused on James Marsters as punk vamp Spike, Alyson Hannigan as goofy sidekick Willow and Anthony Head as stuffy librarian cum demon hunter Giles. Somehow I overlooked the bravura performance from the show's star – mixing defiance, vulnerability and wit with a rare quirky charisma. Gellar has the unenviable task of making you care about someone young and beautiful with superpowers and she pulls it off fantastically.

Joss Whedon has spoken about how the role of Buffy needing a character actress in a leading lady role and he knows he got lucky with Gellar. "She's as good an actress as I've ever worked with," he said. She's excellent playing it mean in the season opener When She Was Bad and her comic timing delivering the numerous zingers the scripts throw her way is flawless.

But it's the Angelus arc, where her boyfriend and closest ally Angel turns overnight into her worst enemy, where she really shines. After taking her virginity Angel is transformed by an ancient gypsy curse into a soulless monster – Buffy's lover is now her tormentor and with her heart ripped out and fed into the meat grinder she's the one everyone looks to for strength. Beauty and superpowers suddenly doesn't feel like such a hot combo.

It's where the show fully hits its stride and shows its storytelling chops for the first time. We're taken into dark territory (notably in the storming Angelus-centric episode Passion) and Gellar knocks it out of the park for the entire second half of the season.

Season two also sees the entry of Spike and Drusilla – the British vampire couple who took over Sunnydale's undead community after hilariously ousting the Annoying Little Shit, sorry, the Anointed One in School Hard. The pair are as devoted to darkness, chaos and each other as a couple of circa 86 Sisters of Mercy fans – he, the wisecracking rebel, she, the batshit crazy drama queen. Watching the interplay between Dru, the wheelchair-bound Spike and the newly evil Angelus in Bewitched, Bothered and Bewildered reminds you of Whedon's comedy writing background. Even when it went off the rails in later seasons, Buffy was always a very funny show.

There's also no doubt that Buffy was terrific for women on TV. Whedon loves to write memorable women – this show has Buffy, Willow, Drusilla, Faith, Glory, Cordelia and Darla. Characters like this resonate, and I see Sydney Bristow, Kara Thrace, Sookie Stackhouse and Veronica Mars as lineal descendants of Buffy Summers. The show also launched the careers of writer-producers Marti Noxon (who wrote the central Angelus arc episode Surprise) and Jane Espenson (who went on to write for Battlestar Galactica and Game of Thrones). It's a strong feminist legacy.

What surprises me about watching again is how Buffy feels like quite an old show. The special effects don't age well; the doomed Buffy-Angel romance feels a bit "Twilight" and between them, Giles, Spike and Drusilla get through more cliches of Britishness than a Hugh Grant retrospective.

And yet there is still nothing quite like it. Subversive, funny and emotionally engaging, you could put Buffy toe-to-toe with any of the giants of the noughties – The Shield, The Wire, The Sopranos – and it wouldn't be embarrassed. It began as a turkey and finished a cultural icon. It's a modern TV miracle.