My personal vote is for gyrojet weaponry. (Rocket-propelled projectile. Neat little idea, actually, neglible recoil. They actually exsist, too, the US Army experimented with them in the 1960s or 1970s.)
Lasers are still a possiblity in atmosphere. The range at which they would be effective would be reduced greatly, though.
The Helios and Cyclops are too powerful for atmospheric use. If you accept that the Helios is more powerful then the Harbinger (and it is, 3200 damage versus 6800), and the Harbinger's equivalency to TNT is 5000 megatons, a Harbinger is vastly more powerful then any current nuclear weapon. 10 or 15 megatons is enough to destroy a fairly large city, so one can imagine that a Harbinger or a Helios is gonna leave one helluva crater. The Cyclops is about two-thirds as powerful as a Harbinger, which makes it only slightly more manageable.