This can be anything from Hyperspace in Star Wars, Warp Drive in Star Trek, travel through the Warp in Warhammer 40k or anything else.
I’ve always liked “slow” FTL travel, where going a few light-years still takes a few days or so. I also really like travel through an alternate dimension like in 40k, Event Horizon, Witchspace in Elite Dangerous.
I wanna know your favorite versions, or do you prefer stories that obey the laws of known physics, like the Expanse or Rimworld?
Mind you it’s a scale. Star Trek is harder than Star Wars, which is harder than Farscape. I think the hardest sci-fi out there is going to be The Martian. We could plausibly fly an Ares mission as depicted in the book, the main technology handwave is the lack of thin, light, radiation shielded fabric for EVA suits and Hab canvas.