Sometimes all the planets align and you get a movie that hits all the right notes.
Casting, script, direction, lighting, props/costumes, score, editing/pacing etc. A great movie is the sum of all these parts.
Although I'm not in the industry some of my friends are and usually they say when you see a good to great movie, it is usually when the studios/ producers don't get involved and trust the Director and the Writer's visions.
Has there ever been a perfect movie?
I think this is all a matter of opinion and what you as a viewer take into account as being important. I can say my favourite movie is probably Raiders of the Lost Ark. I love action movies and this one is a smart, entertaining ride all the way. It is fantastically shot and amazingly cast. The pacing of the movie works wonderfully and I'm invested in all the characters. Is it a perfect movie? Well it's as close as one gets I think.
Some of the others I think are up there are The Terminator, Aliens, The Dark Knight, Momento, Fight Club, Blade Runner, Interview with a Vampire and the Empire Strikes Back. But of course I love Sci Fi and action and these are probably not the same for everyone.
What is the most important element in making a classic motion picture?
I'd have to say it's the Director. He is the one who pulls it all together and leads the orchestra. As you can see above from my favourites, Directors like Spielberg, David Fincher, Niel Jordan, Chris Nolan, James Cameron and my favourite Ridley Scott put together amazing pieces of cinema.
What do you look for in a movie that in your opinion would be a classic?
The story is always the most important part I guess, but if you are going with the word "look" then I guess it is how the visuals are repesented on screen. Take Blade Runner for example, just a visually stunning expereince from start to finish and was done on a very tight budget and without CGI etc. that is used today. The Director is the key to this so when it comes to what I look for, it is the actual world in which the story takes place makes a classic... and that goes for all considered a "classic" likes those you mentioned such as Gone with the Wind, Citizen Kane etc.
I'll use... ahhh... Tron: Legacy as an example... not an amazing story, but the visuals are so good, you can excuse the script to some extent, and walk away happy you saw the film...
Of course these are my opinions as you asked OP.
Cheers!
GS