The Karnov definitely stands out for cheap (PV) and utility. Especially for off-board use, cheap is best.
The only problem is fluff. The Karnov artillery is a rumored variant "no confirmed sighting exists." Gameplay wise, the Karnov is an excellent choice, story wise, it's doesn't fit.
Chaparral, Padilla and Thor are Comstar only. So scratch those off the list.
That leves:
Snipers - Ballista, Marksman
Thumper - Thumper AV
Long Tom - Mobile Long Tom
The Long Tom wins with those. Blast radius and damage, it's a winner. For 28PV (versus the lowest non-Karnov of 20 PV), that blast radius and damage is well worth it. I might even take one Mobile Long Tom over three Karnovs (I'd use the Karnovs for direct fire over the mobile long tom, since they are harder to hit and three of them can take more hits, but I'd never take the Karnovs over the Mobile Long Tom for off-board, blast radius is everything there).