At least early on, Willow is a Subverted Trope - quiet, plain (in a Hollywood Homely way), not especially strong, and easily the most intelligent and intellectual of the teenage characters. In fact, Oz probably counts too.
Not coincidentally, Willow's hair got distinctly more and more red as her character became more assertive and less nerdy and quiet.
However, the stereotype was completely avoided when Willow went berserk at the end of Season 6. Her hair turned black. So the only time she is seen as a truly badass redhead is via the alternative-universe Fetish Fuel Vampire Willow.
Willow was always fiery, she just lacked self-confidence. As far back as the Season One "I, Robot, You Jane" she had no problems attacking Moloch (a demon in a robot body strong enough to easily knock Buffy around) with a fire extinguisher.
Castle: Castle is surrounded by fiery redheads (with varying degrees of redness): his spacy but tough mother, his sweet daughter who isn't afraid to tell him off when necessary, and his police detective partner who is a BadassAction Girl with a gun.
Commander In Chief: Vice-President Mackenzie Allen was encouraged by everyone to resign when death by aneurysm got a hold of the President. Instead, she took the Oath of Office, won the country, and was generally a spitfire who shouldered the responsibility of the Presidency without getting drunk on power - and God help you if you stood in her way. Fiery Redhead, indeed!
CSI: Crime Scene Investigation: Catherine Willows borderlines this, as a strawberry blonde. It's not deep red, but probably got enough red mixed with the blonde to count. And she's got the personality to go with it.
CSI: Miami: Horatio Caine, sometimes. You definitely don't want to get on his bad side. He'll hunt you across however many countries it takes, as the Mala Noches could attest. He didn't exactly seem to have trouble getting women, either, though they kept getting killed for some reason.
Days of Our Lives: Melanie Jonas (formerly Melanie Layton Kiriakis). Despite being downright saccharine at times, if you get on her bad side, she'll really let you have it. Also, since she's about to gain her sixth love interest in less than three years, she definitely qualifies as an example of Heroes Want Redheads. Her Parental Substitute Maggie also fits this trope.
Defiance: Irisa Nyira, played by Stephanie Leonidas (who is not red-headed in Real Life). This leading to this comment IMDB: "Is it wrong of me to find her more attractive as an alien than the actor behind it?" and the reply, "Proof that redheads are just better?". Irisa is possibly also an example of the "Big Eyes" part of trope Big Eyes Little Eyes.
Fraggle Rock: Red, with her extreme tomboy nature, her...very high self-esteem, and her dominant personality (which itself causes numerous conflicts), plus her own explicitly stated desires ("The Tale of the Triumphant Princess" anyone?).
Glee: Inverted Trope: the only redheaded character is the super sweet Emma. Every other female character (except Brittany and Tina) is extremely fiery.
Law & Order: Special Victims Unit: When Assistant District Attorney Casey Novak isn't on the scene with guns blazing and taking a hands-on approach to the cases, she is in softball batting practice or bicycling to work.
My So-Called Life: Central character Angela Chase evokes this trope by dying her hair "crimson glow". Former best friend Sharon Cherski is a lot calmer and more manageable than Angela, but she still easily fits the trope. Same goes with Hallie Lowenthal, the series' resident adult Fiery Redhead.
Revolution: Emma in the episode "Home". She wasn't a combatant like most other characters, but she didn't hesitate to get in Big Bad Monroe's face and call him out on his actions. She also attempted to make an honest appeal to his better nature. It didn't work, but he did admit that he wants to be the person she remembers so badly. She was his girlfriend at one time, so it gave them a special connection. Then, when he tried using her as a hostage, she didn't panic. In fact, she ended revealing that she gave birth to his son at some point in time and sent him away. Before she can tell him where, she got shot and killed. She had a major impact on Monroe from beginning to end.
The Sentinel: We have Naomi Sandburg, who seems like a peace-loving ex-flowerchild, until you threaten her son; Jim's ex-wife Carolyn; and Megan Connor, the Antipodean Amazon.
Smallville: Tess Mercer. You do NOT want to get in her way or get her angry.
Spellbinder: Ashka, the callous, mean, chronically double-crossing and power-hungry bitch.
Star Trek: Deep Space Nine: Major Kira Nerys. Former Bajoran resistance fighter, current second-in-command of Deep Space Nine, and with an always fiery personality. She once beat the shit out of a serial killer while the equivalent of nine months pregnant.
Star Trek: The Next Generation: Dr. Beverly Crusher - do not piss her off or you will find yourself dropped by Klingon martial arts faster than you can say 'hypospray'.