For me, the "most essential" is Citizen Kane.
I can understand how some may not see what the big whoop is, but for me, Kane is the movie that taught me how to watch movies. Granted, I liked it on first viewing, loved it on my second, and really saw it as a masterpiece only by the third viewing.
With that third viewing about thirteen years ago, it was like a switch going on in my head, and then I just "got it". I didn't only "get it" concerning Kane, but that third viewing also helped me to connect to other movies that I had seen previously, but didn't see what the big deal was about (Chinatown for example)
I think one of the problems with Kane is that for too many people their first exposure is in some sort of (invariably overanalyzing) film class, instead of as something to be sought out and enjoyed. There is quite alot of baggage to being the "official" GREATEST MOVIE EVER MADE.
Citizen Kane is one of the densest, most fully packed movies ever, and it takes a few viewing's to really see everything.
Also, my pick for the "most essential" movie that few people have heard of, would have to be The Sweet Smell of Success. What an awesome movie.
