Lest anyone doubt the magnetic pull Nirvana's music still exerts, the groundbreaking Seattle trio's latest, MTV Unplugged in New York, debuts at No. 1 on the pop chart seven months after lead singer Kurt Cobain's suicide. The 14-song acoustic album was preceded by little fanfare other than a Buzz Bin clip for ''About a Girl.'' But as one of Cobain's last and most revealing documents, Unplugged hardly needed any added push. Look for the chart-topping Nirvana to outlast this week's second-highest-ranking new entry cult metalheads Megadeth (No. 4) but not necessarily classic rockers Aerosmith (No. 6) and Tom Petty (No. 8), who should remain holiday staples.