http://www.native-languages.org/most-spoken.htm
and the most-spoken in Mexico.
I think the title (and to a slight extent the text of the article) make the language sound like a rare curiosity but it's actually a major cultural phenomenon. Grabbing some numbers (which are always subject to definitional quarrels), I think Nahuatl is as common in Mexico as Tagalog in the U.S. (by absolute number of speakers) or as Chinese in the U.S. (by proportion of the population).
Among many other things, our words for tomato, avocado, chili, and chocolate all derive from Nahuatl.
(Edit: of course the social role of indigenous languages in Mexico is different from that of Tagalog or Chinese in the U.S., and the main indigenous languages there are strongly associated with rural and semi-rural communities, as this article points out... which might mean that city-dwelling Mexicans might have quite a bit less contact with Nahuatl and Maya languages than city-dwellers in the U.S. would have with Tagalog or Chinese.)