No particular order. Also, it’s movies that I watched, can’t speak on essentials that I might be missing.
It’s kinda hard to make a list on essentials tho. Because your personal taste obviously plays a big role. I can’t see my girlfriend liking more than 10 percent of those…
Schindlers List
Gladiator
No country for old men
The grand Budapest hotel
The big Lebowski
The Fellowship of the Ring, The Two Towers
Star Wars (the original one)
Requiem for a dream
Pulp fiction
The good, the bad, the ugly
The lives of others
La vita é Bella
All quiet on the western front (1930 version)
The dark knight
The Truman Show
2001: Space odyssey
Alien
7 Samurai
Princess Mononoke
Trainspotting
Boyz N the Hood
Scarface
The Godfather 1, 2
The Matrix
Clockwork Orange
Shutter Island
Kingdom of Heaven
Wolf of Wall Street
Honorable mentions because they are popular and everybody always talks about them (I like a lot of them, too. Don’t consider them essentials tho):
Inception
Interstellar
Fight Club
Harry Potter
Return of the King
Rest of Star Wars (whatever people consider the good ones at last)
"Should have seen" is strange, I'll go with titles that, if you are into cinema probably you have heard about?
Citizen Kane
The Seventh Sigil
Apocalypse Now
Vertigo (Any Hitchcock movie really)
Seven Samurai (Any Kurosawa movie really)
Pretty much anything from Buster Keaton
Charlie Chaplin (I guess Modern Times)
The Godfather part I & II
Taxi Driver
On the Waterfront (Peak Marlon Brando stuff)
Some Truffaut stuff (I guess "Day for Night" would be the most relevant here)
Close Encounters of the Third Kind to get pretty much everything interesting from the Spielberg side of things
Man with a Camera (ok this one is not something I espect any conema lover to know, it's a very early montage wonder from Russia, always good to get back there and get reminded progress doesn't always go forward)
La Dolce Vita
2001: A Space Odyssey
Shining
A Clockwork Orange
Reservoir Dogs
Lost Highway (The better Mulholland Drive)
Blade Runner
The Matrix
Star Wars
There's some good newer stuff but it's much less "popular" so it wouldn't make sense to expect anybody to know them. Very little new stuff looks like has the staying power to be relevant years down the line.
I guess The Lord of the Rings? But I consider it more of a great book that got a quality adaptation than a ground breaking stepping stone of cinema.
Just off the top of my head: Alien and Aliens are wonderful, Apocalypse Now needs no introduction, Interstellar, 2001: A Space Odyssey, and finally Oppenheimer, which is one of the best movies ever made in my opinion (what can I say, I'm a sucker for an incredibly well-told story).
Krull is one that stays in all my libraries. It's so obscure yet has names like Liam Neeson, Robbie Coltrane, and David Battley. It was my dad's favorite movie.
Think what you want, this movie has layers of depth to it. Every time I watch it I feel like I learn something new about a character and why they are the way they are.
A bunch of other people have mentioned Ghibli movies and since I'm in the middle of a binge through every Ghibli movie I think I'll recommend one that I hadn't seen before a few days ago: Only Yesterday or Omoide Poroporo.
It's Isao Takahata, not Miyazaki, but it's easily my favorite Ghibli movie and one of my favorite movies of all time. It feels so real and relatable, the whole movie is essentially a really slow-paced series of flashbacks to the main character's 10-year-old self and every detail is so well-thought-out and interesting.
Very worth watching, although I'll mention as a disclaimer that all the friends I was watching it with thought it was super pointless and boring.
The original Jumanji.
The first Jurassic Park.
Rush Hour.
The Sound of Music.
Spirited Away.
Alien.
The Dead Poet's Society.
The Matrix.
Happy as Lazaro.
The Dark Knight.
The Godfather.
2001: A Space Odyssey
The Lord of the Rings (Trilogy).
Seven Samurai.
The Terminator.
The Lion King.
Both versions of House on Haunted Hill (evolution of the campy horror genre)
Both versions of Black Christmas (see horror tropes become them and the retelling)
Jason X (Jason the undying killer but in space!)
Teeth (if you needed a movie to really drive home that SA is bad)
Trainspotting (if you needed a movie to really drive home that drug addiction is bad)
The Room (a detailed instructional of how not to write a movie)
Super Mario Bros 1993 (watch as both the main characters appear to become more drunk as the story goes on because their actors actually were) .... And the new one I guess. It's ok but not as entertaining on a meta level
DOA Dead or Alive (possibly the most true to source video game movie and a fun martial arts movie)
Patch Adams (if you can watch the whole thing twice, you're better than me. That third act is brutal emotionally for me)
Slaxx (do not read anything about it. Go in blind. Enjoy the layers of what the actual fuck is going on)
Dungeons and Dragons 2000 (about as accurate a oneshot as I've ever seen. Also, Jeremy Irons being peak Jeremy Irons)
What the bleep do we know (Science! Physics! Learning!)
Wicker man (Cage version during the height of him taking literally any role to pay off debts. Watch him overact and punch a woman in a bear suit)
Romeo and Juliet (decaprio version. Shakespeare but in modern day Miami. They do not update the language to current English.)
I've got more but that should be good for a few days worth of watching.
All Quiet on the Western Front (2022) - not entirely faithful to the novel, but this version is brutal in its depiction of the war. I do think removing Paul's short return to his village hurt the story. It's interesting to compare this one to the 1930 version
Spaceballs (1987) - Mel Brooks doing what he does best.
Some I haven't seen mentioned:
-The Best of Youth
-Enter the Void
-Call Me by Your Name
-Nocturnal Animals
-The Secret Life of Walter Mitty
-The Hunt
-Moonrise Kingdom
-Mother (Korean movie)
-The Baader Meinhoff Complex
-City of God
-Snatch
I know the franchise has been on shaky ground lately, but I really think everyone should watch the Infinity Saga, in order, at least once. 23 movies that tell one vast over-arching story, featuring some of the best god-damn superhero casting ever seen.