Metallica - The Call of Ktulu
I'm also a huge fan of the instrumental break in Master of Puppets, but it's not a full instrumental so I don't know if it qualifies.
I think this type of music just needs vocals. I can't cite much in the way of true Dream Theater instrumentals that I like. But they write such long, technical, instrument-focused music that it's hard for me to differentiate something like Octavarium or, say, one of their various medleys from 'instrumental' music. LaBrie is there, but he's more...strategic, I guess. Not a 'lead' so much as just using his vocals as an instrument that is secondary to pretty much everyone else in the band.
I'm also a big fan of 'concept albums,' where an instrumental may not be that impressive on its own, but as part of the whole, it's fucking great.
Right now this thread has inspired me to listen to Leitmotif (dredg). Might as well be one giant instrumental (technically it alternates tracks with/without vocals). How about opera singing, does that count? No one knows what the shit they're saying, anyway.
https://www.youtube.com/watch?v=un027bv0yPs#t=80
Can I have chanting, at least? :/
Eluvitie - Elembivos