My Man Godfrey was my introduction to Lombard, and I fell in love with her at once. Nothing Sacred features some of her best shit. (I especially like her silent monologue after March knocks her unconscious.)
But I'll always keep a candle burning for Twentieth Century. It may not be her best role, but her contest with Barrymore to see who can out-ham the other is one of the orgasmic joys in the history of screwball comedy. The way they play off each other is a constant delight.