Being strategic and “safe” I would personally choose a specialty or smaller website over a commonly used one. It’s really hard to get away from people’s biases on a popular site. I want to ensure I wouldn’t be running into blind bias from the interviewer, and instead focus on the best practices the site is using.
That being said, I’m also a bit of a contrarian and like questioning assumptions, I also use the interview to test the hiring company and potential team mates, so divisive sites work well for that. Like Facebook or Craigslist. If a “UX Designer” can’t see past Craigslist’s visual design for it’s usability and simplicity, that would be red flag for me. Critiquing failures of iOS usability is great too. If a hiring manager is blind to failures of a loved product/site, or the successes of a hated site, do you really want to do UX and/or design for them?