Koyaanisqatsi - First saw this on a double feature with the Valley Obscured By Clouds (Pink Floyd soundtrack). Came out in '83, not exactly a documentary, it was as far as I can remember the first feature length movie I had seen entirely without dialogue (although the very different Disney animated classic Fantasia has very little dialogue). I've seen it a few times since, but not for a while. I've never found the collaboration of director Godfrey Reggio's insight (helmed two other films in the so called Qatsi Trilogy, the half his life until 28 spent in a contemplative order was ideal preparation for a wordless film), DP Ron Fricke's pioneering technical innovations in time lapse photography (later directed similarly non-verbal films Baraka and Samsara) and the alternatingly haunting and hypnotic score by minimalist composer Philip Glass to be less than enthralling. To this day, I've never seen anything like the work of Reggio and Fricke.
eXistenZ - Caught the last half hour at a hotel while travelling recently. Didn't like it the first time I saw it, but I'm a big Cronenberg fan, and thought I'd give it another try. It has a virtual reality theme (a contemporary of The Matrix and The Thirteenth Floor, all from '99, earlier movies with a related theme include Strange Days from '95 and Brainstorm from '83 - more recently, Inception), and piqued my interest enough to watch the rest on Netflix streaming. Not as good as Videodrome or Scanners, but perhaps in the next tier within his body of work.