Thin Lizzy's Whiskey in the Jar. Instantly recognisable from the first reverberating note.
I don't entirely agree with RR's rule, mind - I reckon intros just have to be fabulous. Check out the dripping chords and the bass that kicks in and hooks you before the drums and the twitchy guitars catch you in The Rapture's Out of the Races and onto The Tracks. The slow build-up, layer on layer, is immensely exciting. Same with the Wilderness of Manitoba's Summer Fires.
Obviously, I've deliberately chosen slightly more obscure tracks to make my point. I don't think they have to be that famous. Possibly that says more about my taste in music than anything else, mind