For me, hands down, it's the Fuel Rod Cannon and Needler. I find it very easy to blast away stupid annoying Rangers with a Needler. For Grunts and Hunters, it's gotta be the Fuel Rod. For Jackal Snipers with Focus Rifles, let's just say I haven't managed to find a favourite... :\
Keep in mind, the weapon has to be with infinite ammo but NO BOTTOMLESS CLIP.
[Edited on 11.23.2011 12:07 PM PST]