uh i'd say multiplayer is generally more popular than single player.
the legend of zelda: ocarina of time, largely regarded as THE best game of all time, sold 7.6 million copies. the best selling zelda game is twilight princess, which sold 8.39 million copies if you add both the gc and wii sales.
call of duty black ops 2 sold 24.2 million copies as of november 5, 2013. pretty much everyone who plays call of duty plays online almost exclusively. the campaign is awful, and everyone i know goes right to the multiplayer.
i would say that the numbers show that from 2 popular franchises of different games, multiplayer is generally more popular. of course there are exceptions such as super mario bros., which has sold 40.23 million copies when combined over multiple platforms, but it's highly unusual for single player games to outsell popular multiplayer games. most mario games have not sold as many copies as cod black ops 2 has, just to put things into perspective.
links i used to prove i'm not just making the numbers up
http://en.wikipedia.org/wiki/Call_of_Duty:_Black_Ops_IIhttp://vgsales.wikia.com/wiki/The_Legend_of_Zeldahttp://vgsales.wikia.com/wiki/Mario