Type of thing you have never seen before or since? Movie that is truly different from anything else? For me I have to say its Apocalypto- by Mel Gibson. Its a masterpiece IMO. If it had been done by Scorsese or Spielberg it would have garnered more praise. Cos of Mel Gibson it was passed over. Pity.
Man bites dog would be up there. Same with Cannibal Holocaust. Naked lunch, perhaps. Hard to pick. I would say that people saying Avatar, or really any other mainstream movie should watch more movies... There's nothing unique with those movies. Apart from showing off new tech (3D for Avatar). Avatar is Pocahontas in space, for instance.