Dva.
At least I have a chance of killing Genji, Pharah, Junkrat, or Reaper before they do any real damage. Dva just drops hers and runs off and you're forced to clear the area.
My greatest joy was watching Dva drop her alt and run for cover, but I froze her in place with Mei. Instead of shoving an icicle in her skull, I just posed and let her ult kill her.