Ooops -- is this supposed to be about bass riffs on the guitar, but not riffs on the bass guitar -- ??

But you did say "I am thinking rock"... :)

When thinking of the Dead, the first bass riff that come to my mind is Phil's thundering underpinning of the lines "Coming/coming/coming around" in "The Other One."

(Hope that's not a mondegreen -- I've never worried about understanding all those nutty lyrics, as many times as I've listened, as many versions as I've witnessed.)