Well, to pick a single song is next to impossible for myself. So I'll break it down some.
It'd definitely be a choice between older Judas Priest, obviously from their Sad Wings of Destiny or Stained Class album. I really like Victim of Changes, Dreamer, Epitaph/Island of Domination from their Sad Wings of Destiny album. However, their song Beyond the Realms of Death is simply amazing, especially the lyrics. Not to say the lyrics from Victim of Changes or Dreamer don't inspire me. It's also Robert Halford's amazing vocal range combined with his vocal control. Anyway, there are many more Judas Priest songs I could add to this list, seeing as Judas Priest has been around since the 1960s.
The next band up would have to be Dream Theater. Although I didn't like Charlie Dominici as a vocalist, their first album "When Dream and Day Unite" was still amazing. Though my favorite album (which tends to be everybody's) was Awake. With the songs Voices, Lie, The Mirror, Scarred, The Silent Man, just amazing. James LaBrie as a singer is phenomenal and on par if not beyond Robert Halford's capability. The song writing of Dream Theater shouldn't be taken lightly either. The amount of excruciatingly hard work and dedication they put into their songs, it'd be a shame if they weren't as long as they are. Which I could never complain about their song length. When I'm listening to Dream Theater, I never want their songs to end.
Anyway, I'm actually kind of tired, so I might update this a wee bit tomorrow. I'm always up for serious music discussion (even though I doubt I'll find many people on NB that would be on par with my tastes. Especially since most people on NB are younger than myself).
Edit: Though, let me add real quick. If you're a real fan of Instrumentals, I highly suggest you check out the band Liquid Tension Experiment (headed by John Petrucci of Dream Theater). What they're capable of truly is insane.