4 out of 4, which seems a slightly unintuitive scale

Finally, there’s a level 4 sting, which is as bad as it can get. Schmidt knows of only three critters capable of inflicting level-4 suffering: the warrior wasp (Synoeca septentrionalis), a two-and-a-half-inch-long black bug found in the tropics; the bullet ant (Paraponera clavata), also tropical; and the tarantula hawk (genus Pepsis), two inches long, which Schmidt can find in his yard in Tucson.

-- The Straight Dope

btw, this image is a legit snippet from the wikipedia page Schmidt sting pain index although it doesn't make clear that Schmidt didn't get stung on purpose for science.