There's no way I could figure out what "THE BEST" intro track is, but regardless of that, I will say the track that popped into my head almost immediately when reading this post is "Deliberately Limited Preliminary Prelude Period In Z", from Helloween's Better Than Raw. One of the things that shapes one's perception of an intro track is how well it segues into the next track, and this combination of classical orchestration and electric guitars works perfectly as a lead-in to "Push", which is quite possibly still the heaviest song Helloween has ever done.