FLAC is first choice, anything MP3 in descending order of quality, then, anything else.
No, I can't tell the difference between FLAC and MP3, 320. But, I have a bit-perfect and playable backup of my CD, and it gives me a warm fuzzy feeling knowing that my source isn't my limitation.
Maybe in the next 10 years they figure out how to augment human hearing. I can check out my old FLAC files and hear what I've been missing. MP3 People get to buy "The White Album" again. Besides, drive space is cheap.