I have seen and rated ~1000 movies on imdb including many of the Top 250, but I haven't seen The Godfather either... yet.
But also haven't seen any James Bond movie (although I recently started watching the first two), Lord of the Rings or Harry Potter...
I really dislike mainstream movies, but I have problems with art movies too. Try watching Stalker without falling asleep. It's an interesting movie, but I can't get the hype. The same goes for 8 1/2 or Persona.
Citizen Kane on the other hand, CF, is IMO a great movie.