Not to rain on any parades or anything, but I would disagree with the notion that the best of the stuff being mentioned in this thread was not recorded, and is lost to time because no one remembers it.
I disagree because if you're in the business of writing songs and making records for a living, your better ideas are going to be written down, given a demo or scratch recording, or in some other way cataloged for future use, even if that use includes being re-purposed into another song or song idea. Those ideas which come and go in the minds of writers are so fleeting, or can be so instant and as a result so easily forgotten, you're going to make note of them. Witness those dozens of tapes Lennon left behind which made up the "Lost Lennon Tapes" series...there were hundreds of individual recordings, everything from simple ideas to fully demo'ed songs. And as wasted as John, or Keith Richards, or any of them were at any given periods of their lives, there always seemed to be some way for them to record ideas which turned into legendary songs. Those like Keith waking up in a daze, recording a riff on tape then totally forgetting about it, only to have it eventually become "Satisfaction" when he eventually played back the tape and worked on it.
Those professional (successful) songwriters all seemed to have a system of jotting down ideas they think are "good", whether it be a portable recorder, a journal/notebook, or a cocktail napkin from the bar they happened to be in when the idea hit them. With the modern age, the fucking iPhone will probably advertise a way soon to tap into your imagination and record your subliminal thoughts...all for a nominal fee, of course, and a contract which involves signing away all your rights to privacy and confidentiality where your best thoughts get recorded and stamped with the Apple logo and TM.

Any artist, any medium: It gives a certain romantic "lost to time" element to consider or speculate on all those masterpieces that only existed in the imagination of the artist, never to see the light of day, but ultimately what's the point in getting sad over what wasn't recorded when there is so much to learn about what actually exists for us to hear, see, feel, etc.?