Don't all Brachydios weapons have it since that's kind of his thing? I don't know for certain I'm just guessing.
Yes, all weapon types have a Brachydios line, and all of them have slime element on them (except for the bowguns, which rapid fire/siege mode slime shots, i believe).
Quite a few of the weapon types also have a weapon line that has extremely high raw damage and high slime element that needs to be awakened. Like the uragaan hammer and the diablos lance.
Slime element is far and away the strongest element in the game. There are obviously going to be situations though where other weapons perform better. Like with Hunting Horn, where which songs they'll need is often the deciding factor for what horn they bring in a group.
So...does that mean the slime weapons are all the best weapons outside special circumstances?
if I am using a sns...is the slime one simply the best, or will I still have reason to use others outside of very specific situations?
Well the thing is that like a Barrel Bomb, Slime explosions do fixed damage, so if something is weak enough to a particular element it's still going to be better to use that element over Slime if you can keep your attacks at a steady enough pace.
Shine on, embrace and deny me. Turn on, Wheel of Time. Shine on, you've burnt me, now guide me. Weave on, Wheel of Time.