Wine is tied with marble for 'most important' resource at the initial level of the game to speed up city growth and produce more citizens to sell into worker, researchers, or conscript into the military.

Crystals overtakes wine as the 'most important' resource at the mid/end level of the game due to its ability to speed up those obnoxious research times (4 days!?!) when you research glass out of the science tree. By the time you have glass researched (its halfway down ) your cities will have grown to the size that you've already stuffed as many as possible into your academies, mines, and saw mill so you have have them milling about reproducing and paying taxes. The time at which you reach that point is different, however, since it depends on how large a military you maintain. Larger military means larger upkeep, means more citizens needed.

Oddly enough Wood and Marble both tie for second most important at the mid/end level because upgrades to your buildings start to get ridiculously high.