Well, the most popular format is MP3. You can't go wrong with that format. It will playback in iTunes, Spotify, Windows Media Player, iPhones & Android phones, and tablet devices. All MP3's include album cover art, artist name, track name, album and lyrics if available.
If you are looking for high fidelity lossless audio formats then you should download either WAV, FLAC, or AIFF. You may also select to download OGG format which is not lossless but good quality audio. OGG is similar to MP3 quality but less popular.
To learn more about Audio formats please go to Audio.