One of the strongest entrances I have ever witnessed on film was R. Lee Ermy’s portrayal of Sgt. Gunnery Hartman in “Full Metal Jacket”. He quickly establishes who is the alpha male in the room and just doesn’t let up.
Tom Cruise in Magnolia
Mongo in Blazing Saddles
Seriously? It has GOT to be Bad Nun returns from Black Narcissus.
Sorry, better video here:
YEEESSSS !!!
Three, actually, and although his second entrance when he crashes the Normans’ banquet with a dead deer slung around his shoulders was memorable enough, my all-time favorite is right after the ambush in the Greenwood (cue the brilliant Korngold score here) when he gazes down on a defeated and furious Guy of Gisbourne and a stunned but intrigued Lady Marian and in a wonderful combination of “F*** YOU, Dude” and “Oh you kid!” bows and delivers my favorite line in the entire film, “Welcome to Sherwood, My Lady!” Brash, flirtatious and simply irresistable.
Can we count TV shows? Martin Sheen’s entrance in the first episode of The West Wing was pretty damn good.
How about the entrance which is also an exit? I nominate the arrival/departure of Tuco in The Good, The Bad, and The Ugly.
Oh, bravo. A wonderful scene from a wonderful movie.
I believe Meryl Streep still deserve even an honorable mention on her entrance as Miranda Priestley because of the fear of the employees scrambling in the office and the anticipation of the audience watching.
I don’t know about the greatest, but I always thought Rachel Ward as Meggie Cleary coming down the stairs for her grandmother’s birthday party stunning. Especially Father Ralph’s reaction, as he realizes the little girl he loved as his own has grown into a beautiful young woman.
James Coburn in the Magnificent Seven
I’ll go with entrance/exit of the well girl in The Ring.
Ole smoothie Frank Booth.
Best wheelchair intro by Dr. Strangelove.
First appearance by spooky black widow Christina Hasslag in The 4th Man.
Ed Harris goes for a coffee in History of Violence. ("JOEY")
Spooky woodsman in Rituals
Onstage naked blonde temptress in Clockwork Orange.
Members of Talking Heads appearing one at a time in Stop Making Sense. (awesomely cool)
Robert Duval making himself at home in Apocalypse Now. (crummy video quaiity) (and technically, he does appear earlier, in the helicopter, playing his reel to reel tape of Flight of the Valkyries.)
Peter O’Toole was mentioned earlier, but not sure if his holy appearance in The Ruling Class was.
Sleazeball producer in Boogie Nights.
Hoochy koochie dancer in Metropolis.
First sight of Conan in Conan the Barbarian - we see the child Conan enslaved, sent north and chained to the Wheel of Pain, where he spends years building muscle.and then eventually Arnie raises he head and stares grimly into the camera
https://youtu.be/z5KYZ74OAak?t=144
Over here, control-z chimes in wrt favorite action or fight scene.
I don’t know if this quite fits the OP’s criteria, but that one scene is the first time, in multiple movies, that we ever get to see why Darth Vader is so feared.
In a way, we were reintroduced to an iconic character.
From this thread:
I don’t know if it qualifies for “greatest”, but Rose stepping out of the car in Titanic definitely is in the running for “loveliest”.
Since you brought up Dr. Strangelove, I must point out it also has one of the greatest character exits in movie history.
https://www.youtube.com/watch?v=snTaSJk0n_Y
Roman Polanski in “Chinatown:”
Michael Biehn in planet terror.