Psycho would be a good choice since it fits the theme, is extremely influential, has a pretty interesting movie structure, and for god's sake guys, how have you not done a Hitchcock film yet?!
I also really feel like you guys have gotta do another Carpenter commentary, and with so much of his stuff being horror I think now would be as good a time as any. Like I said, They Live would be great because of the abundant social commentary on display. If you wanted to go more pure horror In The Mouth of Madness would be a good choice.
I hadn't considered Prince of Darkness......if you think you can explain what the fuck is going on in that movie you guys are welcome to go for it.
Someone mentioned Scream, I think I would almost rather you guys do that then Cabin in the Woods, because in many ways Scream is doing the exact same thing as Cabin in the Woods, except it got there 20 years earlier, and actually works as a scary horror film. Also you've kinda already done a Cabin in the Woods discussion.
Also, I second Blair Witch Project. If you're going to have a discussion about found footage horror, it makes a lot of sense to do the movie that started the trend. If you wanted to do a modern found-footage, I think Rec is a far more effective and interesting example than Paranormal Activity, both in terms of concept and execution.