Off the top of my head, I'd say the 2nd most popular cult film is "Rocky Horror", which basically can still be called "cult" despite its popularity. Sure, you've got events and the whole audience participation thing (and didn't they also do a TV version recently?) but it still mostly appeals to a niche crowd of weirdos, partly because it's a musical but mostly because it's pretty bizarre. The average person probably hasn't seen it and if they did, they probably wouldn't like it very much and certainly wouldn't buy into the cult.
Although with the release of "The Disaster Artist", "The Room" has a good chance of becoming way more popular than it already is and I can actually sort of imagine it having more widespread popularity than something like "Rocky Horror".