Knights of Cydonia

This is number one purely because of the riff immediately following the tapping. (I believe its called Rampaging Horsemen, or something) I never get sick of it.

More Than A Feeling

"It's More Than A Feeling!!" C'mon, aren't you feeling it? The chorus riff is effing amazing.

Crazy Train

For the intro riff. Love it!

The State of Massachusetts

Banjo-picking baby!

Lay Down

This is an awesome head-banging riff. Number 5 on my list.

Bang Your Head (Metal Health)

Not really catchy in the sense of the word; but there's no way else to describe it.

Hot For Teacher

Simply amazing riff during the verses. Breaks my heart.

Layla

This version is by Derek and the Dominoes. Sweet and melodic with a hard rock background.

Heartbreaker

Jimmy Page is a fricking genius.

Reptilia

I like the stop-and-go riff after the "strange part of our town." section in this song.