Whether invoked with admiration or derision, Kenny G. is a household name. But while the smooth jazz saxophonist may get under the skin of purists, his gentle, melodically focused playing has made him a mainstay of adult contemporary radio and one of the most popular instrumentalists of all time. THE ESSENTIAL KENNY G., a generous two-disc set, may be the best Kenny G. package on the market. It covers all of his hits ("Songbird"), appearances with guest stars ("We've Saved The Best For Last," with Smokey Robinson), holiday songs, and unusual covers (his version of, believe it or not, Outkast's "The Way You Move," performed with Earth, Wind, and Fire).