I always liked Jade. If you can find a picture of it the guy is giving Linda Fiorentino a hell of a good time 
I like Police Academy 6’s poster for some reason.
Shoot me now.
What about Blazing Saddles?
My personal favourite (which I can’t get for love nor money) is a [Rocketeer art deco poster](http://www.deco-dence.com/ generic.html?pid=55) (about halfway down the page).
if we are talking simple beauty, it is the one hanging on my living room wall:The Sin of Nora Moran
In posters, and in movies, you can’t go wrong with The Iron Giant
All the monster movies are great -Dracula-Frankenstein etc.Film Noir are very well done,as are westerns & Robin Hood w/ Erroll Fyynn. Reissues that are nice are Psycho & Rebel w/o a Cause. BTW, an original King Kong poster sold for about $800,00 or so at Christie’s a few years ago. At the time it was the most paid for a poster.
This one from The Truman Show. I always loved it.
This is what I had on my wall when I lived in student housing.
It’s not terribly romantic, I’m afraid.
Later on, I also added this classic shot from Pulp Fiction.
The chicks really loved me back then, I’ll tell you that. 
Alien. Brilliant poster.
Polish film and theatrical posters are great, but may frighten young children. Check out the works of Wieslaw Walkuski.
SmackFu is right, though. Be prepared to lose and arm and a leg.
I always liked Hoosiers. It’s really simple but says so much without really saying anything.
I agree the Rocketeer is a good one. Another I loved for the art design was Costner’s Wyatt Earp which was a horrible movie, I’m afraid.
Midnight Cowboy is a classic, but will depress you every time you look at it.
The stylized comic-book feel of the (Warren Beatty) Dick Tracy teaser posters was fantastic. As were the individual band member posters for U2 - Rattle & Hum
It all depends on what sorts of movies he likes though, I mean, he might find the poster for Faster Pussycat, Kill! Kill! to be more meaningful. Hard to say.
By the way, actual theater movie posters are bigger than standard posters you can readily buy a poster frame for. You can buy theater poster-sized frames at a specialty store but they’re pricey. If you’re not prepared to drop a lot of money on an actual theater poster and framing you might want to stick to the films that have mass-produced posters in “regular” poster sizes. Off the top of my head I can think of Jaws, The Godfather and Pulp Fiction. You might have luck finding more recent stuff like Star Wars or LOTR type stuff. Also culty films have the posters reproduced a lot. I seem to remember Eraserhead being a big one when I was in my teens. Rocky Horror as well.
Oh! One I just thought of. I understand there was an early poster for Fargo that was a really beautiful needlepoint with a grisly murder scene in a small section of it. (Maybe it was a bloody axe or something – I have seen it but it was so long ago I forget.) Apparently the sweet little old lady who had been comissioned to do it was horrified when she found out it would be seen by millions of people she demanded it be pulled. If you could get your hands on one of those it would be amazing!
My vote would have to go to one of Hitchcock’s movies. I especially like North by Northwest, but Rear Window’s poster on the first page is pretty cool.
Anatomy of a murder is very cool
Only one I’ve ever owned is an original for Apocalypse Now.
I’m embarassed to say, but the one from Titanic], showing Rose and Jack embracing with the bow of the ship beneath.
In my long-gone teens, I had a “Night of the Living Dead” movie poster that was great! I wish I still had it. Showed a zombie gnawing on a bone and all that cool stuff.
My boyfriend stole it…uh…I mean, got it for me right after the movie came out. An original! Well, maybe not original, but definitely new, at the time.
My mom threw it away (along with about twenty others) after I turned twenty. Makes me mad just thinking about it.
I had a “Bless the Beasts and Children” poster, as well, but it wasn’t as cool. Wish I still had it too, though. (sigh)
Now there’s a funny thing, my mental image of the poster from ‘The Graduate’ had Dustin Hoffman and Ann Bankcroft’s leg in totally different places . . .:
http://www.foximas.com/gb/the-graduate-poster.html
Another not mentioned that sticks in my mind is ‘Clockwork Orange’.
spellingerror, some great posters have been mentioned already and I don’t know what movies your brother loves, but I would strongly suggest checking eBay and some online poster sellers for foreign posters. European and Japanese posters are typically much cooler looking than American ones, they’re treated more like art and not just a sales tool. One of my favorites is a Belgian Dead Ringers poster I got on eBay. There was also a beautiful Italian Blue Velvet poster I saw at a convention years ago, but I didn’t buy it because it was huge and I lived in a small apartment at the time.
Knowing that your brother is an illustrator steers me towards recommending posters made by the designer Saul Bass. Bass worked extensively with Alfred Hitchcock – and as you can see by the number of people here who like Hitchcock movie posters, he definitely strikes a chord – and has been called “the inventor of the modern film title.” In addition to posters and titles, he also did storyboards, and is often credited with carrying out the shower scene in Psycho. Definitely a major figure in the graphic arts world.
But yeah, an authentic one-sheet would set you back dearly.