What music format is the best?

Unless you're an audiophile with $500 headphones plugged into your iPod, you probably won't hear a lick of difference among the alphabet soup of formats. Even when listening on your home stereo system, the more important choice is between lossy versus lossless compression.

Lossless compression preserves all of a song's audio information, so the compressed file sounds just like the original-- and stays fairly large, around 10 megabytes per minute. Windows Media Player, iTunes and RealPlayer all give you the option of ripping CDs to a lossless format, such as WMA; FLAC and Monkey's Audio (APE) are common open-source lossless formats.
Lossy compression creates much smaller files (about one megabyte per minute) by removing the parts of the sound you're least likely to miss. MP3 is the most widely supported lossy format, but newer codecs such as AAC and Vorbis provide better sound than MP3 at similar file sizes. Note that you can't go from lossy back to lossless--those crystalline highs and gut-shaking lows are gone forever.
Most online music stores lock you into one lossy format that has copy protection encoded into it, limiting the number of computers or type of portable player or software the song will play on--potentially annoying in 5 or 10 years when your equipment changes and your entire Def Leppard collection is suddenly worthless.

The best way to future-proof: Own, don't rent. Buy used CDs on Amazon or eBay, rip them to a lossless format, and resell them. Sure, this eats more hard-drive space, but that's cheap. Then just make lossy files in the best format your current portable player supports. Alternatively, rip to MP3 at a high bit rate (320 kbps), and store those discs in your Mom's attic.

Jonathan Coulton is PopSci's contributing troubadour as well as a professional musician and software engineer. Give him a listen at jonathancoulton.com.