Fairly straightforward.
John Williams’ score for Star Wars, especially the main title, the Darth Vader march, and the “Acid Klezmer” Cantina Band are all iconic.
For lesser, I’ll go with the bluegrassy goodness of Ballad of Serenity (the theme to Serenity for anyone not familiar).
What are some of your sci-fi/fantasy faves musically speaking?
“Also sprach Zarathustra” for 2001.
Seconded
Williams has done so many good ones. I’m partial to the Jurassic Park theme.
Not being a major sci-fi fan, I can only add the Blade Runner soundtrack by Vangelis and the theme to Close Encounters of the Third Kind by John Williams.
Long ago, the theremin sounds in The Day The Earth Stood Still were great to hear.
And I’ve always been a sucker for the original Star Trek theme by Alexander Courage.
Nothing beats John Williams’ work on Star Wars.
Second place goes to “All Along the Watchtower” from BSG.
My husband (Andy L) and I jokingly considered having the Doctor Who theme for our wedding march.
I really like the new music from Doctor Who–not the theme, but the “the Doctor is doing something awesome” music.
What? No mention yet of the closing music to Silent Running?
Excuse me while I wipe away a tear…
The theme from the orignal Battlestar Galactica (The music starts at 0:36, after the voiceover). Just simply stirring music.
Third mission of the RTS game Homeword uses Barber’s Adagio for Strings sung to Agnus Dei.
The entire score to Conan the Barbarian.
Good one! (I am such a sucker for that movie)
I’m going to go ahead and nominate the Eagles’ Journey of the Sorceror, which any true sci-fi fan should recognize as the theme song of a certain TV series.
Benson, Arizona from Dark Star.
“The Creation” from Bride of Frankenstein
“Opening Titles/Outer Space/Radar” from The Day the Earth Stood Still
“The Conversation” from Close Encounters of the Third Kind
“The Asteroid Field” from The Empire Strikes Back
“Theology/Civilization” from Conan the Barbarian
“Battle in the Mutara Nebula” from Star Trek II: The Wrath of Khan
Children of Dune, Inama Nushif, Cleansing of the House montage - YouTube Inama Nushif from the Children of Dune miniseries, easily. Absolutely haunting.
No contest. The music Grieg wrote for Peer Gynt. It’s all good, including stuff you don’t know you know, like “Hall of the Mountain King,” but it includes possibly the single most beautiful short piece of music ever written.
How did I not know there was a CoD miniseries? Is it worth buying?
I like the Dune one that precedes it and COD, though you’ll definitely find fans who’ll debate the merits of both. The COD miniseries basically combines the Dune Messiah and Children of Dune books, with obviously a lot of cutting and reworking. Personally, I think it works, most of the acting is good, and I own both series 
Pretty much all of The Nightmare Before Christmas. Elfman’s theme from Batman is great, too.
Star Wars and Jurassic Park have been mentioned, but I’ll go with John Williams’ Superman theme.