I don't really understand what's happening here. Are we supposed to identify a specific riff or are we just saying, "Yea somewhere in this song I love a particular riff. You Dr. Garbanzoure it out."
Nah, we're talking about, like, the lead riff in a song. Easily identifiable, usually right up front; the one part that a really drunk dude would remember if he couldn't remember any of the words. The three songs listed above are good examples.
For years I've considered this to be one of the absolute best opening riffs ever. Even if metal isn't your thing, you have to admit it gets you pumped up!