Headphones

Important to mention that it's not just the audio file that contributes to what you're hearing.

It's the listening environment, your headphones/speakers, the quality of the recording/post production, and your overall familiarity with the small details of the song.

If I'm listening with my IEM's while walking my dog outside and there's environmental noise and the music isn't getting my absolute undivided attention, then I probably won't hear the difference between MP3 and FLAC.

But when I'm giving the music my undivided attention and listening in my silent room with my HD800s, I can absolutely hear the difference with well produced albums. It's very marginal, but it's noticeable.