As someone who very rarely goes to theaters (I didn't even know about the other MCU movies until I ran across them on redbox. My last theater trip was in 2010 for the Dawn Treader), and who doesn't often rewatch movies I've already seen (at least not for a few years), I have to say that The Avengers became the first film that I watched twice in-theater (two different cinemas; the first time in 3D). That's kind of a huge deal for me. I was even tempted to see it a third time, but I'm restraining myself until I can get the DVD.
Edit: Ironically, (since I've not even seen any of the X-Men films-yet) it was actually all the public buzz around X-Men: First Class (back in 2011) that made me decide to see what superhero movies I could find on redbox, causing me to stumble across a little something called Iron Man... and that post-credit scene... Yeah...