The way I make my characters' dialogue unique to them, is by considering what makes them unique. But that's best explained by examples. So here we go.
Female character is nervous, worries about everything, and is a bit old fashion. "Oh, dear no. I understand equality and all that, but really? You want to be out there in the war? Bullets flying everywhere, people being shot? Think of your poor dear mother. Do you think my heart can take knowing her only daughter is out there?"
French playboy talking about helping a woman relax. "Oh, but mam'selle. It would be a sin to not offer."
A gamer complaining about rush hour traffic. "This is worse than loading screens!"
A posh gentleman that feels like he should have been born in Victorian England. "Is this what society has come to? Vagabonds prancing about, declaring themselves gentry. Politicians speaking ill of women. I'll hear no more of it. Not a word. Good day, sir."
A shy nervous wreck that apologizes for everything. touches teapot "I'm sorry!" friend gives them a curious side glance "It's okay. Sorry, I. This isn't. I'm sorry. Really. Sorry. Uh. I should go."
Look for things the individual character values, or things that make them stand out. Give them verbal quirks as well as personality ones. Gender can play a role in this, but isn't usually the defining factor. You can have a tomboy that talk like 'one of the boys', or a more genteel man speaking in a far more reserved or indirect manner.
Typical traits to pay attention to are: cultural heritage (Scarlet o'Hara in Gone with the wind), socio-economic standing (posh rich girl versus trailer park girl of same geographical area), archtypal behaviour (tomboy versus goth girl versus geeky girl).
Note that you should avoid stereotypes as best you can. Some stereotypes are unavoidable, and often true. But if you have one representation of that social group, you could be called out on writing from stereotypes. Easiest way to get around that i to only call on stereotypes you understand, and then contrast them with others from the same social group (example: the flamboyant gay, just have three gay men and have each act differently; no one will deny that some gay men act like that, but it's far from all).