I would like to update the list, because when I made that post, I didn't have all shotguns.
1. Graal remains on top, because it is still fun personified. You can't beat the kachunk, or how good headshots feel. Especially when that headshot is past a Guardian's shield. I love it when I manage that. On host, off host, up and around the host, a bit of experience goes a long way. I especially love that gun on the KroSol, the BatSol, the Talon Merc and the BatSent.
2. The last Rare weapon I unlocked was the Raider and after a few runs, I figured it out and couldn't help but be amazed by its stopping power. With the proper use, it can tear the enemies a new one. It doesn't have the stagger provided by the Graal, but there's no need to stagger the ones that are dead. I use it with the TSol, the HSol and the KroSol.
3. Piranha is the third one, because sometimes you just need to go blamo all over the place, up in their face, everywhere. It's a fun gun, obviously designed for the brawling bunch, which is why I give it to my KroSol and my Krodept.
I'll mention the Disciple and the Claymore as well, even though they didn't make the top three, as well as the Wraith, because it still is a good gun.