My vote goes to the Ursa - its the one of the most untapped potentials in terms of in-game usage... Runner up would be the Dragon.
Both the Ares and the Erinyes are barely interesting to use from a campaign design perspective, other than an excuse for the player power trip, or top tier enemy ships - but when you think about expanding mission design outwards from "doing pew-pews", e.g. by thinking in terms of synergetic formations and campaign progression mechanics, there is no need to use them.
The Ursa, and by expentsion of the Medusa, Osiris and Amun, actually present an interesting angle of attack with their turrets - the PVN bombers with 2 turrets each more so than the GTA ones - as they could make better non-player combatants. The Ursa's high HP also make it a less annoying escort prospect, I think - or a particularly enduring threat.
The Dragon could use a "bolder" interpretation, but that is purely aesthetics.