As a genre fan, Pacific Rim's opening is awesome.

It establishes the setting, events, and tone in a way that's incredibly badass and leads up into an epic kaiju fight before the title screen even rolls.

Princess Mononoke's opening.

Obviously, the best opening is the first minute and 43 seconds of Spaceballs. Nothing has ever been made, not ever will be made, that is as epic, inspiring, and critical to what a movie is trying to accomplish as that scene. I stand in awe, all these years later. Bravo, Mel! I say again, bravo!

Glad to see that no one has responded since my post. We are all in agreement with it, I see.

Army of the Dead. Very fun and crazy opening.
Friday the 13th (2009). I rewatched this movie during Halloween and it's better than what I remember.
All of The Lord of the Rings movies.
X-men(2000)
Mortal Kombat (2021). Too bad the rest of the movie wasn't nearly as good.

Bambi: Opening credits and then a morning in the forest.
Star Wars: A New Hope. The title crawl and then the ships in space is so iconic and awesome.
The Lion King: Circle of Life is iconic
The title sequence of Spider-Man 2. I feel it's the best title sequence of the trilogy, especially with the paintings that recap the first film.
Harry Potter and the Deathly Hallows Part 2: Quick recap of the end of Part 1, followed by incredible music (Lily's Theme) and haunting imagery at Hogwarts. Then when the title shows up with the music getting louder and more intense, it's perfection. Then, there's a beach grave. Another recap from the end of Part 1. No dialogue for the first two and a half minutes of the film, and I don't think I'd have it any other way.
The Lord of the Rings intros, all three of them.
There's many other amazing or at least great movie openings. But I felt these ones are the top.

I'm going to have a bit of a variety.

Rear Window 1954 - the hot weather has everyone with their windows wide open (this was before widespread AC), and Jeffries (Jimmy Stewart) has a broken leg and nothing to do but be bored and watch all these stories going on in the buildings behind his rear window. This is before Jeffries begins to descend into madness, and (unless you know the answer) you're not quite sure if a murder has happened, or if it's all just in his head. One of the best setups to one of the best films in history. Hitchcock, like the directors of his time, tended to avoid exposition, "Show Don't Tell" was seemed closer to a law than a guideline; these films usually didn't hold your hand - some of Hitchcock's later films did. Hitchcock shows you what's going on, and doesn't hold your hand a great deal. But back to the scene, it sets up the beginnings of a lot of different short stories that occur in the apartments outside the rear window, some of these will collide later, but you don't know that yet. Anyway, I love this intro.

Casablanca 1942 - it really sets up the backdrop of all of the politics and the core struggle that's going on around the pub (Rick's Cafe). The whole film is a microcosm to show the French resistance against Nazi Germany, and the other players involved. But also, people desperate to get out of Vichy France. Rick (Bogart) represents the US, Renault (Rains) represents Vichy France, and Strasser represents the Nazis. Like Rear Window, this is a "Show Don't Tell" used in place of exposition, it's not holding your hand but giving you the visuals of what's happening, and you form the image of what's happening on your own.

Dawn of the Dead 1978 - While the action remake from 2004 has a great intro, I think the opening to the 1978 film is the greatest horror film opening of all time. VERY relevant to what's going on with Coronavirus today. Basically, the zombie apocalypse could have been handled, zombies are easy to kill. Conspiracy theories about zombies make people doubt the zombies are actually undead, they think they'll get over it. No one wants to destroy the heads of their loved ones. It wasn't the power of the zombies, but the fucking up of humanity, that brought the zombie apocalypse to the four corners of the world. This is only one of the openings, the second opening has the other main characters, but it's probably a little too extreme for this forum as heads explode, there's yelling of the N-word, and extreme racism, murdering of police officers, etc... This is kind of an interesting intro as it delivers exposition, but not very on the nose exposition; it gives you the pieces of the world, and through the film it solidifies in your head. This film isn't about humans vs zombies/zombie apocalypse, but humans vs humans causing the zombie apocalypse. The main characters are intelligent people in a world of the stupid, insane, and undead. This film is kind of a two scene opening, so, what's shown here is really only half the job, the exposition, the next scene shows the dangers of the world and extremes this film will go to (as I mentioned, the next scene is probably too extreme for this forum, at least, not outside the NSFW section). 

Scarface, 1983 - this delivers the political backdrop to the crime wave that produced Tony Montana. This film is a remake of the original Howard Hawks Scarface from 1932. The film sets up the sort of character Tony Montana is, and gives us all the background information we need on him prior to the rise and fall story that follows. Exposition delivered in the form of a police interrogation of a Cuban refugee. I'm not saying exposition is bad, otherwise I wouldn't put this as one of my favourite openings (mostly because of the humour, though); it's more a result of dialogue becoming more important during the 1970s, so exposition became more heavily used to deliver more complex narratives. Nowadays, people need their hands held, otherwise they lose track of what's happening in films, so films before the 70s (outside of highly cliché genre films) tend to be harder for more recent audiences. Unfortunately, it doesn't show the assassination scene, which, IMO, does the rest of the work for setting the tone for the whole of the film not quite covered in this scene.

X-Men First Class 2011 - I usually don't like blockbuster films, but this is one of the exceptions - the scene is vengeance against Nazi pricks. It establishes the film as a reverse reimagining of the gothic classic Frankenstein, and it's put very bluntly in this scene. Shaw, like Dr. Frankenstein, couldn't control the product of his ambition, and his monster set out on a mission to destroy him. The exposition delivered in the form of a deadly barfight in three different languages between the monster and associates of Nazi Dr. Frankenstein. The founding of the X-Men is really just the outer pulp, what really matters is set up here, in this scene, the core story of the film, and what the ending is all about.

Goodfellas 1990 - this one has narration over sequences, after showing a scene from later in the film (which Scorsese does in other films) where they butcher Billy Batts. The scene talks about how much Hill (Liotta during the adult scenes) loves being a gangster, and how the goodfellas all love being gangsters. Exposition is very deliberate in this narrative, and masterfully executed. As mentioned in other films, the butchering of Billy Batts (which happened just before this) sets the tone for the extremes this film will reach... but it actually goes beyond it IMO, the tension gets absurd in the later portions of the film. But this scene really sets the lighter and cooler side of being a gangster with a little dash of the brutality of it all. Also, don't worry, Frank Vincent (who played Billy Batts) kills Joe Pesci back in Casino :D

A Clockwork Orange deserves to be here, but the "no nudity" rule probably makes it against the rules to show the opening, but it's one of the most famous intros in film history. So I'll go to my favourite recent Kubrick inspired film:

Hereditary 2018 - The atmosphere and cinematography very much resembles that of The Shining, and the Dollhouse is basically the model hedge maze that transforms into the actual thing. The actual plot details aren't revealed, but you get a strong sense that this family is not in control of their destiny, but some other sinister force is at work. It jumps into the exposition in the form of a Eulogy.

Children of Men 2006 - this opening establishes a world on the brink of chaos, like Dawn of the Dead, this is an apocalyptic film. This film is known for its long shots and VERY visceral violent scenes. It is a beautiful film, one of my favourites.

There are more I like. But what I consider a good film opening is one that can give a general sense of the atmosphere and what the film is going to be like within the first few minutes.

Lots of good ones mentioned already. One I didn't see was Infinity War. You got Thanos beating the shit out of Hulk and killing Loki. Was a phenomenal opening sequence.

Akira (1988) had a fantastic opening and a great soundtrack, very cerebral. Still unmatched in the anime category.

Hands down The Dark Knight. This movie's opening is perfect.