Ok. Richard Strauss may have done that intro in Also sprach Zarathustra, but this is great:
 
Deep Purple - Knocking at your back door
https://www.youtube.com/watch?v=CrOdBxXOQi4
 
I don't know why I like it - maybe that's why I still like it.

I've always liked the intros to:
 
Mountain - "Mississippi Queen"
https://youtu.be/fMXeZWv6AS0
 
Beatles - "Sgt. Pepper's Lonely Hearts Club Band (Reprise)"
https://youtu.be/Sc7gicwg43U
 

 
 

 
 

Alan Parsons - Sirius
http://youtu.be/aBQalkIeE7s
 
Perhaps inspiration for John Williams written about 65 years before Star Wars.
http://youtu.be/Jmk5frp6-3Q
 
Gamma Solar Heat
http://youtu.be/eo4lcQUWNUU

Leizer
Deep Purple - Knocking at your back door

One of my favorites too. 
 
But the little synth pieces Steve Miller did were great too.
 
 
 

Ain't talkin bout love.
 
 

I like the intro to Ozzy's Diary of a Madman, Saxon's Crusader, and Judas Priest's Electric Eye, AKA The Hellion.

Maybe it's just the Canadian in me, but I always crank this intro up when I hear it. After the guitar part, I turn it back down again
 

At three and a half minuits its longer than a lot of songs but the "intro" of "Revealing Science of God" (Side one of Tales from Topographic Oceans) by Yes. Andersons chants build tension underpinned by Wakeman's synth work until Squire, White and Wakeman release the tension at 3:33 in one of my favourite moments on any record.

KenB123
I've always liked the intros to:
 
Mountain - "Mississippi Queen"
https://youtu.be/fMXeZWv6AS0
 
Beatles - "Sgt. Pepper's Lonely Hearts Club Band (Reprise)"
https://youtu.be/Sc7gicwg43U
 

+1
 
or is it +2
(because I agree with both)