Things that come to mind when I think of riffs,
Black dog, Sweet Child O Mine, Sweet Home Alabama, enter sandman, Walk this way.
That type of thing.

But the show seemed to include.
Smells Like Teen Spirit, Stairway to heaven, Not riffs imo.

Then there are the ones I am stuck on such as
Smoke on the water whilst it's a riff it's chords so I don't know. Same goes for Seven Nation army.

Though this definition would say they are.

'A guitar riff is defined as a cluster or sequence of notes or chords that is used in a song. It can be repeated over and over again, or it may only be heard once in, say, a lead guitar solo. Riffs can be minor decorative elements or they can be the basis of a song'

What's your opinion awimb? and whilst at it favourite riffs?