IMHO, the best opening guitar lick ever was George Harrison's scorching slide on John Lennon's "I Don't Wanna Be a Soldier."
Heart first introduced Barracuda at a free open air concert in Boston in the late 70s. I was there. Very cool opening lick. Whats also cool is that the actual feedback during the lick was made into "studio" feedback on the recording using guitar
Nevertheless, best opening lick is Van Halen's "Running with the Devil"