I personally try to find 320 kbps mp3, the quality satisfies me a lot. There is m4a, amazing quality as well, but most audio programs don't support it, so it's a no for me (m4a are smaller than mp3 files, that's a plus). The best are obviously .wav and .flac, you get the best quality out of those. And again audio programs support .wav's, but most dont support .flac, so I try avoiding it. The easiest to get are obviously mp3's, everyone uses that format because every single device/program is able to open it. My advice would be that if you don't use audio programs and you want to have more disk space, then go for m4a. But if you're into programs, then choose mp3/wav. From my personal experience I can tell you that not many people use flac, whenever they get a flac file, they usually convert it to wav.
I hope I helped at least a bit