There are a lot of acids in the world, many of them present in biological creatures. Humans, for instance, have hydrochloric acid in their stomachs. What I'm wondering here is just how nasty it could be if one such creature could use such an acid as a weapon. I suppose this is a two-part question: just how nasty could that acid be, and what would the creature have to be like in order to use it effectively?
I'm envisioning a creature (not necessarily intelligent) that spits acid at its prey, hopefully at a distance of more than a few feet, but feel free to poke holes in that assumption if you come up with something creative. Ideally, it should be able to use acid like what one might see in answers to this question, but that might not be realistic. If it comes down to a choice between two (or more) acids that would be better against different targets (flesh versus metal, for instance), I'd like to see the choices outlined, but you can assume this creature would be using its acid for hunting, not destroying buildings.
You can include suicidal examples for the sake of a thorough answer, but I'm looking for something that can actually survive using its own acids without serious side effects. Surviving its own acids being used against it, like if it was fighting another of its kind, would be interesting but is not required (although it's obviously detrimental if it can't eat any prey it used its acid on, so it should at least be able to survive ingestion in small quantities).
For the purposes of this question, I'm focusing on how the creature could plausibly do this naturally, without magical or technological assistance. An answer with evolutionary reasoning behind the development of such a trait would be a bonus, but genetic engineering is allowed if necessary.