i would say and this is me the maliwan hell fire smg it is mostly found in playthrough 2 The most powerfull is Water gun....Easy to use....and lots of fun ;)
That depends on the character you're playing: Sniper - Volcano sniper rifle
Soldier - Glorious Havoc/Glorious Machine Gun
Siren - Hellfire SMG and/or Double Anarchy SMG
Berserker - Maliwan Crux and/or Vladof Hammer shotgun
Still, for any character - close to the end of the game - you'll be much happier with Eridian weapons (also any character can use those weapons I've mentioned, but they won't be as great of a match as with the character they were meant for).