Using boxofficemojo.com's Estimated Tickets Sold data for R-rated movies, these are the Top 50 as of September 2013.
For example, The Godfather is 51st for Total Domestic Box-Office Revenue for R-rated movies. But it jumps to 2nd in Estimated Tickets Sold when adjusted for inflation (it was released in 1972). By adjusting for average ticket prices for the year of release, these are the R-rated movies with the highest attendance in theaters.