I'll start:
Favorite: Scarab.
I really like the higher fire rate over the scar, and the increased mobility. I use the extended mag. as the only attachment.
Least: K-Volt
Its probably the most frustrating thing ever for me. The damage is far to low to be competitive in a shoot out scenario. Plus how slow the projectiles move and the average fire rate pretty much add up to a hard to use gun to me.
Most would suggest the damage needs to be buffed, but i would rather see it hit more so maybe increase the fire rate and the overall flight speed of the projectiles.