Remember the '60s? Hair was growing long, revolution was in the air, and...Barry Manilow hadn't yet terrified the world with his arena-size schlock. Having dispensed with the 1950s on his last album, Manilow has moved on with The Greatest Songs of the Sixties, wrapping his tenor and a string orchestra around Bacharach tunes, the Beatles' ''And I Love Her,'' and other '60s ballads. No arguing with the choices, but as usual, Manilow positively buries the songs with his excessive emotionalism. Best moment by far: ''When I Fall in Love,'' a schmaltz classic worthy of the Barry treatment.