I swear by the Purifier. It's from one of the warbonds, but I don't remember which one offhand. It has about a one-second chargeup, then deals a bunch of damage in a small AOE. The only real downsides are the travel time of the projectile, and that you have to watch if enemies get close to you, because it's easy to blow yourself up with close-range panic fire because of the AOE.
Edit: I double-checked. It's from the Polar Patriots warbond
http://i.imgur.com/VeNBg.gif http://i.imgur.com/gd5jC8q.gif
http://i.imgur.com/PKIy7.gif http://i.imgur.com/3p29JqP.gif