It's her battling skills.
See, everybody else just plain-out sucks, has a Pokemon and doesn't use it, or is named Officer Jenny or Nurse Joy.
For the anime, Jessie's IQ is actually higher than average, and has above average battling skills.
Above-average battling skills? I like Jessie (and I find Team Rocket infinitely more interesting than all of the "twerps" combined), but I don't see how her battling skills in the anime are meant to be above average, since her Pokemon invariably get beaten to a pulp not just by Pikachu, but by practically any Pokemon that happens to be defending justice at the time.