My Nominees:
Dr. Zachary Smith - Lost In Space
Ensign Pavel Chekov - Star Trek
All those great 1950s babes screaming in The Hideous Sun Demon and The Phantom Planet (a pivotal moment in the plot!) and The Curse of the Faceless Man and tons of others, and you choose a couple of guys???
Any of the screams in the first segment of 2001: A Space Odyssey
Piercing monotone scream? Newt, from Aliens. That kid could bring it.
Sorry. I was going for humor. For babeage screams, I’ll throw in Dale Arden from Flash Gordon.
C’mon, guys, you know who it is: Fay Wray, in the grip of the King.
Agreed. But I gotta give props to the falling stormtrooper in Star Wars. You know the one: “AaaaaAAAAaaahh!!!”
Not sure if it originated in that flick, but I hear it in a lot of movies.
Kkkkkhhhhhaaaaaaaaaaaaaaaaaaaaaaannnnnnnnnnnnnnnnnnnnnnnn!!!
Godzilla, in the original B&W flick.
For sheer volume, at least.
Let’s not forget Kirk in that Twilite Zone ep. Earned Shatner the title of “The Male Fay Wray.”
“IT’S ON THE WING!” AAAAAAAaaaaaEEEEEEaaaaOOOOOOaaaaaaaag … !!! urp
That’s the one. Thanks Terminus. I always thought there might be an interesting story behind that sound clip.
Nathan Lane in The Birdcage, when he pierces the alien toast.
That might be one of the coolest pages I’ve ever read.
And I desperately want to believe that Sheb did the scream. We’re distant cousins.
I’d go with that, not just because of the acoustic quality, but because of the way Shatner screwed up his face and shook right before. Priceless.
Apart from that, does Fullmetal Alchemist count? Whoever does the voice of Ed Elric in the dub can shriek.
After they try to transmute their mother in the first episode and Ed sees the result, the scream he lets out is friggin’ awesome.
Jerry Colonna was famous for his (seemingly) 5 minutes yells back in the 40’s.Does that count as a scream?
Also,Vincent Price (and most of the cast) in the Tingler
It was in “Alien”, and it wasn’t an actual scream, but the advert for the movie: In space, no one can hear you scream. So, it’s kind of a Zen thing. The non-sound of someone screaming. 
Not Sci-Fi, but does Fantasy count? All those “Arabian Knights” movies where someone meets a horrible death (often ordered by the despotic sultan); and as they’re sawn in two, crushed beneath a lowing ceiling of spikes or have boiling lead poured on them, they scream (nearest possible spelling:) AYE-EEEEEEE!
I’d LOVE a .wav of that.
NOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOoooooooooooooooo!!!
How about Taffee O’Connell in Galaxy of Terror? Pretty good for a weather girl.
*Please note: Galaxy of Terror is an extremely obscure and silly sci-fi/horror film. View at your own risk.