My opinion as an archivist is that for textual information there is no more efficient method than microfilming. Microfilm is stable through many temperature ranges (assuming polyester base), lasts about 100 years before you need to recopy it, it is self evident in use, if all the microfilm technology were lost tomorrow we could figure out how to use a roll of microfilm in 30 seconds, it is small, an entire year's newspapers can take up maybe 6-7 rolls of film, which is less than half a foot of shelf space, and it doesn't take any special technology to use it like CDs/DVDs, in a pinch you can read microfilmed material with a flashlight and a white wall.

So yeah, the answer is probably surprising if you're younger, but polyester microfilm is probably the best thing humanity has ever invented when it comes to preserving textual information. It's been widely in use in information science since the 1930s.

Sorry, but this question is poll-type (asking for thoughts about the "most" something subjectively) and a "throughout history" question, neither of which is allowed in the rules. If you're really interested in sorts of information preservation, you could suggest it to /u/caffarelli as a Tuesday Trivia feature topic, which is where we put some of the "throughout history" questions that otherwise wouldn't be allowed.