There was a thread on Tribalwar with 1.13 million views and over 23,500 posts.
It was called Girls, Girls, Girls.
It was locked because it was causing a server drain every time it was accessed, and replaced with Girls, Girls, Girls, cotd.
On Salon’s Table Talk in the old days (before it became a pay site) it wasn’t at all unusually to have threads go into the tens of thousands of posts. They didn’t report the number of views, at least not as far as I know, so there’s no way of knowing if any of those threads win the all-time views contest.
Ars had a Post 9/11 thank you thread (thread, thread views total) that ended up with about 1.3 million views. After it was locked, most of the images were hosted in an archive that registered several hundred thousand hits on its own. That is the largest non-porn thread I am aware of.
The most views would probably go to some of the messageboards that Fark has linked to due to them having the porn du jour (Paris, Pam and Tommy, etc) but those sites usually crumble from the traffic. Alternatively, the Slashdot effect has certainly driven hundreds of thousands of hits to sites, but I think the numbers are usually less than a million.