the bass riffs to prayer and keep the faith are about as close to perfect a riff as you'll find imo.

the intro riff on dead or alive too.

i think i'll just pick one from every album to keep it simple:

new jersey: keys riff on blood on blood

ktf: aside from the title track the riffs on in these arms

these days: the verse guitar riff on these days and the break riff on something to believe in

crush: captn crash intro

bounce: guitar riff on intro of love me back to life but my favourite thing on the whole album is the vocal hook on the chorus of misunderstood.

HAND: intro to HAND

lost highway: riff on any other day intro

circle: intro riff on wwbtf