Nothing worse than other people's kids. I wanted kids, and I'm thrilled with the two I've got... but I've worked for years to make them into non-monsters. They have manners, they know how to behave, and they speak like small adults... and then we see other kids, and my skin crawls. I want to yell at the kids and punch the parents in the face.
When my son was 4 or 5, we were at the target and some other kid was having a hissyfit in the toy dept about something... and I felt like there was an opportunity. I got about ten feet away from the wailing kid, leaned down and said to my son, "Just so you know, if you ever act like that in public, I will kill you.".... and I said it juuuuuuuust loud enough for the other parent to hear me. I got a dirty look over it, but I made my point (to my kid AND the other parent), and they took their brat elsewhere.