Is ECO still the best reference for all chess openings? Is ECO, Encyclopedia of Chess Openings still considered the best printed reference work for all the chess openings, as it was back in the '80s?
There does not seem to be a consensus on this, but here is a relevant discussion:

https://www.chess.com/forum/view/chess-equipment/chess-opening-reference-books?page=1

For printed opening material (as you requested) I prefer ECO with Chess Informant updates for new opening ideas and analysis:
http://www.chessinformant.org/about-chess-informant/