The song that did it for the group is "Sweet Child O' Mine," which moved up from No. 2 to end the two-week reign of George Michael's "Monkey" (now No. 4). But Gun N' Roses won't have long to savor its latest success, because there is a dangerous challenger poised to take over: Huey Lewis & the News' "Perfect World," which leaped from No. 6 to No. 3 this week and already is No. 1 in radio airplay. ("Sweet Child" is No. 1 overall because of its dominance in record sales.)