At its frequent best, Harry Connick Jr.: A Celebration of Cole Porter, opening tonight at the Nederlander Theatre, pairs Porter's songwriting genius with Connick's superb musicianship, supple, ear-pleasing vocals and a brash confidence that pushes the music from the comfort of classic pop into bolder, jazzier terrain. Connick, with his years on American Idol, movie screens and concert stages, is certainly the most popular interpreter of American standards, and he takes fine advantage of that good will, unafraid to slip in an occasional dissonance or to slow down a vocal like a train creeping to its halt. Where Connick leads, his audience knows to follow.