I honestly cannot remember the worst game I've ever umpired. There have been plenty of them, and I try to forget them as soon as I walk off the field.
That said, if I had to pick a level of play where I've seen the biggest gap in talent between teams (leading to these kinds of blowouts), it's JV Girls Softball. Around here, the better schools have a JV team that would give some schools' varsity teams a run for their money. So when they play those schools' JV squads, the games get ugly quick.
I recall some JV games when I started umpiring softball that never got past the second inning before we reached the time limit (yes, our JV softball games are time limited for this very reason).