Lineage Plasma Cannon (Apocalypse)
The Lineage Plasma Cannon is quite a powerful yet compact weapon, and it can fit on most vehicles, including the Hoverbike and the Phoenix Hovercraft. This weapon is fueled by Elerium and is probably the most cost-effective Elerium-powered weapon.
While a Hawk Air Warrior equiped with three Lineage Cannons can prove sufficently powerful to take down a mid-tier alien craft, its undisputable biggest weakness is its appalling accuracy. And with its impresive range it tends to start firing long before it has a chance of hitting (the enemy at least). It will regularly succeed in doing ten times more damage to the city than the aliens if not handled carefully.
(If I recieved $1000 for every time the hospital has had to be rebuilt from the ground up, by the time I stopped using the weapon I'd never need any funding again. The best way to avoid toppling hospitals and mass shopping mall collapse is to save before every cityscape combat and try to position your forces to fire away from any tall or delicate structures. Also be sure to keep an eye on your elerium reserves, as it can use them up quite rapidly and empty guns are no fun.)
The other option is to only use them up close. The procedure used to conserve the use of the Stasis Bomb Launcher can be used here: disable the hardpoints mounted with Lineage Cannons, get in close, then re-enable the cannons and hit the UFO at close range.