There's not going to be a specific gun that rules over all in PoE. Of course gjallarhorn is gjallarhorn and you should have it equipped 24/7 but there's different shield bosses you have to face and buff that's could give bonus or less damage on a certain elemental. I'm probably going to go all fire elemental for cabal (gjallarhorn, fire shotgun, and voc) for hive, (fatebringer, arc shotgun and thunderlord) for fallen same as above except with gjallarhorn) and for vex (fate bringer, swordbreaker and gjallarhorn