This is a very common interview question, and one that's good to think about beforehand.
I was asked both ends of this question-- which apps/sites I liked, and which apps/sites I didn't, and why. I had recently written a (since deleted) blog post about why I hated reddit's design, so I started there. I talked about their page layout, the clicks-to-content route for each type of content, the multitude of font sizes used, and how I would smooth things out for them. For a site I liked, I spoke about TuboTax-- a company that has mastered UI/UX so well that it's made even the most boring, difficult subject in the world downright easy and fun. I talked about their use of iconography, their quasi-gamification of the income tax experience, and how their sales model has apparently changed over time.
I felt like I did quite well on this question, mostly because I cam prepared and knew points I was going to hit on. For anyone job searching, I recommend taking some time to think about this question before going in. Not only will it allow you to be well-prepared on speaking about apps as a whole, but you'll find yourself picking apart different flows and designs as you go, allowing you to speak about the individual pieces much more easily than otherwise.