The Mossberg 500/590 series recommends itself, on account of the placement of the safety (top rear of the receiver, equally accessible for right- and left-hand use). Due to its popularity, it has the advantage that there's a wide range of aftermarket accessories available for it (in particular the Knoxx SpecOps stock).
The Ithaca model 37 has long been popular with southpaws because it ejects its empties downward. But the new incarnation of Ithaca's been redesigning their website, and the defense models page is currently empty ("coming soon... any month now... honest"). Even if they get round to making a defesnive model, it'll never be as well supported with aftermarket accessories etc. as the Mossberg.