This is a very satisfying intro to intros. I'm ashamed to say I only know well about 1/3-1/2 of the openings to these wonderful, major songs (fwiw "Let It Bleed" is my fave).

The intro to Egg's "A Visit to Newport Hospital" is a rather complex series of riffs (first verse doesn't start till around 2:54), each of which is a subtle variant of the previous and is developed in the sections that follow as well (the opening riff--repeated as part of the outro--may remind you of that of "Smoke on the Water," but it's actually a couple of years older): https://youtu.be/e7_xdm_CJ_k