Well, this is just off the cuff (and I’m certainly no format export), so perhaps not the best format ever invented, but certainly up there. High quality digital audio. No DRM. No compression. Such a great physical format that it has been reincarnated in several forms (DVD, HD-DVD, Bluray, etc). You can pop a CD in your computer, rip it to your hard drive with out any pesky copy protection, re-encode/compress those files into any number of other formats as devices and formats change, etc. If only DVDs and Bluray were so easy to use….
This is why I still prefer to buy a physical CD over MP3s. (Well, it helps that used CDs can be cheaper than their MP3 counterparts at Amazon. If I’ve got some free MP3 credit coming my way, though, I certainly don’t mind using that.)