If you had to single out one movie as the best ever made, which one would it be, and why?

Very hard question because there are so many ways films are good. I don’t think there is an objective answer to the “good/bad” question, but I can give my reasons why I consider it the best film for me.

Vertigo (1958)

Most consider this one Hitchcock’s masterpiece. It came out a few years after Rear Window (1954), but right before North by Northwest (59) and Psycho (60). These are considered his big four. Notorious from 1946 is one of his greats as well by film-buffs. Hitchcock has a lot of greats.

Why do I like it more than any other?

I’ll touch on a few of those reasons:

First and foremost, it impacts me emotionally like few other films do.

But also because it’s like a meditation on film, there are a lot of dreams-like scenes, and the climactic twist in the middle of the film with Hitchcock’s famous “Hitchcock time bomb” plot mechanic is never better executed than it is in this film (IMO). Thematically, the film goes beyond the intellect, and into colour metaphors, there is doubtless a connection between “vertigo” and “vert” (green) which is used as a colour to catch the eye of Scottie, the main character (played by Jimmy Stewart).

It's the sort of film that hits differently each time you watch it.

Is it for everyone? I don't think so.

I don’t think this is a good film for Gen Z, since they are sensitive about age gap romances, and this one features one with a man around 50 and a woman who is 25 and it deals with themes of consent and coercion. There is a lot of innuendo and questionable stuff going on in the background. For instance, in this scene, see if you can pick it up:

Notice the subtle way her dress and undergarments hang in the background - which implies more about what Scottie was up to before this scene.

And there is so much more to think about on just this scene the second time through, once you know the twist. The film is dark, and darker still than it appears on the surface. But also an enjoyable watch at the same time. I probably watch this film no less than once a year. The most recent time was about a month ago.

The film is not for everyone - it’s not as accessible as films like Rear Window or North by Northwest, but it is IMO Hitchcock at his very artistic peak.

Speaking of Age gap romances, this is another one I like (retold in The Simpsons, although with a different sort of a twist on it, they replaced the idea of an affair with father vs teacher as a male role model... and also starred Dustin Hoffman as the voice of Mr. Bergstrom):

Anyway, for a summer flick. I mentioned Rear Window. The film again stars Jimmy Stewart as a photographer with a broken leg. There is a heatwave going through New York City, and everyone has their rear windows open to let the air in. There are all these little stories going on in the different apartments throughout the film while Jeffries (Stewart) spies on them. Again, The Simpsons made an entire episode based on this film (this one even closer to the source material), and Lisa plays Lisa (played by Grace Kelly/Princess Grace). Rear Window is also one of the most widely accessible films ever made, and I think just about everyone who watches will enjoy it during this excessively hot summer.

Incidentally: Lisa's Substitute and Bart of Darkness are two of my favourite Simpsons episodes.

Here's another one I think is phenomenal: The Silence of the Lambs

The most historically important film is probably Casablanca. This scene depicts Patriotism vs Nationalism. It was recorded not long after the fall of France to the Nazis.

I consider Kubrick the best director and A Clockwork Orange his best movie, so for long time I would've said that.
Another top one is Blade Runner, though that has the problem that it's the 90's Director's Cut (and later version) that is the great masterpiece, not the original version.
And yet further candidates for GOAT include Das Boot, Sunset Boulevard, Cabinet of Dr. Caligari, LA Confidential, M, Snatch and obvious ones like Godfather and Shawshank Redemption.

But to name just one as the absolute greatest film of all time, I choose Once Upon a Time in the West.
Building on the Dollar trilogy (which themselves are among the greatest ever), it is the peak of cinematography, sound design, casting and overall storytelling.

Literally watching 2001 right now for the 3rd or 4th time this year (at the part where they're excavating the monolith). I find it's a great film to have on in the background, something to glance over at, and listen to without being distracted. But also something I can watch all the way through it I want. I also love A Clockwork Orange, and everything else he's done from Dr. Strangelove onward; I watch The Shining every year around October/November.

I also really like The Godfather, and Blade Runner → which I recently watched (The Final Cut) along with 2049.

Billy Wilder is another one of my favourite directors, The Apartment is one of my all-time favourite films along with Sunset Boulevard. The Apartment really feels like the inspiration for Mad Men, and Shirley Maclaine's acting really darkens the comedy whole thing in a fun sort of way, giving an alternative perspective to Bud (Jack Lemon).

Casablanca. I wont elaborate.

My favorite movie is "Wolf Children" by Mamoru Hosoda (2012). The best anime ever made, better than any Studio Ghibli or Shinkai movie in my opinion. Ame is a very deep character. Unfortunately the trailers are pretty bad and not very representative for the movie.

My favorite non anime is A. I. - Artificial Intelligence (2001). Written by Stanley Kubrick and directed by Steven Spielberg after Kubrick's death. I find the story about a boy longing for love even better than the artificial intelligence aspect of the movie.

Casino. Story, directing , acting, cinematography, music, art

I don't know how to tackle this question. There are a lot of movies that I enjoy, find entertaining and rewatch but would not consider the greatest ever. I find it difficult to watch really old movies and pretty much the only two black and white flicks I enjoyed are Nosferatu and Modern Times, and the later hade colored segments. A movie like 2001 a Space Odyssey have fantastic visuals, especially for its time, but it took me several times for me to actually view the whole thing since it bored me to death.

A few movies comes to mind, where they might not be the general top of the line but in their respective genres in what they try to archive they are top of the line. The Lord of the Rings still make any more recent fantasy epic look cheep some 20+ years later.

Requiem for a Dream gradually make the watcher feel more and more uncomfortable and is probably the best film I never want to watch again, not much is able to make me feel bad like that movie.

The strive and passion to reach new heights and the price that comes with it is done so fantastically in Whiplash that I forget every other film that explores that theme. 

I guess the greatest film ever need to be great in every aspect movies have in common. Stunning visuals and cinematography, exceptional acting, an engaging story, a soundtrack with distinct emotional carry as well as being influential. And even if it is not a personal favorite and I think another movie did the gangster saga better, it is hard to ignore the master craft that is the Godfather. 

Just for context, my favorite movie is The Prestige. 

Die Hard

Endlessly watchable, with engaging and memorable characters, and great writing, music, action / set pieces etc.