I've been stung by jellyfish, wasps, bumble bees, and bitten by spiders (not big huge ones), red ants, mosquitos and those bitey flies. Oh and the odd cats and dogs.
For me the worst by far was the bumblebee. I put my hand on it not seeing it there and it hurt sooooooo much. Like my hand was on fire or something. I don't really remember completely what the actual pain felt like that as it was years ago but I do remember that it hurt like nothing else and for ages too. Poor thing didn't even mean to sting me, I put my hand on it!
Jellyfish did hurt but not agony - it was only one of those small ones you get. (In France)
Red ants can hurt and itch like mad too.
What's the most painful bite or sting you've ever had?